Expert Responses

There are several factors that determine when a virus is more likely to transmit, and there are a lot of unknowns, even for well-studied viruses like influenza virus. We have obviously not studied the novel coronavirus in all seasons yet. Most seasonal influenza viruses transmit best in cold and humid conditions, so we see outbreaks in the winter in temperate climates such as is present most of the United States. In tropical countries, however, these viruses transmit year-round despite the heat, and we can find influenza virus infections in the summer in the US if we look carefully. Viruses tend to transmit more easily if there are more susceptible hosts, and pandemic influenza viruses, such as the 2009 H1N1 strain, transmit well in the summer months as well as in winter. The major outbreak of the 2009 strain started in August here in Memphis. As we have seen, this virus did not die down in summer months. SARS, a similar coronavirus, also transmitted during the summer. The question now is whether cold weather will enhance transmission during the winter, worsening the pandemic.

We don’t really know the answer to this yet, as it has not been widely studied across different groups and in patients receiving medications or therapies such as steroids, immune suppressants (like cancer chemotherapy or anti-rejection drugs), or monoclonal antibodies for auto-immune diseases. Many viruses do more easily infect and/or cause worse disease in persons who are immuno-suppressed for one of these reasons. However, some infectious agents don’t seem to cause appreciably different effects. The best advice from UTHSC experts right now is to assume you are at greater risk and be vigilant in prevention measures like hand washing and avoiding infected individuals. Since the virus has become widespread in Memphis, you should probably discuss with your treating physician whether modification of therapy makes sense to reduce your risk for a period of time.

At present, children are typically not severely affected. Some children do get a post-infectious syndrome called Multi-System Inflammatory Syndrome or MIS-C that can be quite severe, with inflammation of internal organs. Children can transmit the virus as easily as other age groups. The precautions for this group are the same as for anyone in an outbreak – avoid infected persons and keep their hands clean if they are mobile and touching potentially contaminated items.

Most respiratory viruses, including coronaviruses, can survive on surfaces for periods from roughly a half an hour to a few days. Exactly how long depends on the size / type of particle the virus is contained in, the humidity, the temperature, and characteristics of the surface. Most viruses of this type tend to survive better in low temperatures and in high humidity or wet conditions. Larger droplets with more liquid probably mean longer survival. The virus can be found in the air for up to 3 hours when aerosolized, on surfaces such as stainless steel and plastic for 2-3 days (with a median survival of 13-16 hours), but lasts on cardboard less than a day (median of just a few hours) and on copper surfaces for only a few hours maximum. This would seem to imply that common areas like metal shelves in grocery stores should be high risk for transmitting virus and should be cleaned regularly, while items like mail packages or packaged foods are of little risk. Although we know that the dominant mode of transmission is respiratory droplets causing person-to-person spread, the advent of winter with colder temperatures may increase transmission from cold surfaces.

Currently, persons who spend 10-15 minutes within 6 feet of an infected person (with or without a mask) are quarantined at home for 14 days with daily monitoring by public health officials for development of symptoms.

Older persons (those over 55 year of age) and persons with chronic medical conditions including high blood pressure are at higher risk of hospitalization and death with risk increasing with increasing age. More than 80% of deaths have occurred in persons older than 65, but numerous deaths in healthy adults have also been seen. The only age group that appears to be spared significant illness so far is children 18 years and under. Link

The COVID-19 test currently in use detects the nucleic acid (e.g., RNA) genome of the virus and should be positive during the infectious period, but then should turn negative after the virus in eliminated from the body. With similar viruses, we often see a period (of days to weeks) after the acute infectious phase where the test remains positive, but we think it is just the remains of inactive virus or its components before they are cleared by the immune system. In some cases, if the level of nucleic acid in the sample is right at the threshold of detection and repeated testing is taking place, a person can be positive, then negative, then positive again, clouding the picture. In a research setting, scientists will be able to detect the presence of antibodies to the virus in the blood for months or years after infection and determine who was infected in the past. These sorts of tests are not useful for determining current infection, however.

We struggle every year to get the public concerned enough about influenza to get their flu shot and to take appropriate self-protective measures, despite the seriousness of this infection and the 10’s of thousands of deaths in the United States alone. However, the public is used to influenza and has adapted to its annual circulation – it is a normal fact of life. The novel coronavirus is, in a word, novel, which means it will take some time to understand and adjust. We now know that more than 3% of identified cases are fatal, which is 100s of times worse than influenza. It is likely that that number is overinflated by 3 to 5-fold since we are only testing a fraction of cases for infection, but it is still likely in the estimation of UTHSC experts to be much, much worse, particularly in sub-groups like the elderly with chronic conditions, than seasonal influenza and we have now seen 100s of thousands of deaths in the United States – more deaths than influenza causes in a typical 5-10 years.

There are several common human coronaviruses which cause the common cold, and in some cases, pneumonia in small children (e.g., coronavirus OC43). In the last 20 years, three new coronaviruses have emerged from animals which cause severe pneumonias – SARS, MERS, and the novel coronavirus (technical name SARS-CoV-2). The severe lower respiratory disease syndrome caused by the novel coronavirus that emerged in 2019 is called COVID-19, which stands for COronaVIrus Disease in 2019. The novel coronavirus can cause inapparent and mild infections, which would not necessarily be termed COVID-19. Currently, testing for common human coronaviruses (like CoV OC43) is not done routinely because of the cost and the mild disease they cause. The PCR test that has been recently developed for the novel coronavirus and COVID-19 is specific for that strain and will not detect common human coronaviruses.

We have very limited data so far on outcomes from pregnant women with COVID-19. Similar viruses, including influenza, SARS, and MERS, cause more severe disease in pregnant women and frequently lead to spontaneous abortion or premature labor, leading to fetal demise or premature birth. As such, UTHSC experts currently recommend that pregnant women consider themselves high risk and take all necessary precautions to avoid infection.

Ultraviolet light (UV) can inactivate the novel coronavirus and similar viruses, but not very easily. It requries prolonged exposure at close range and at certain wavelengths, and the UV light can be blocked or disrupted by a number of factors. Because of the strict requirements, it is not deemed to be a practical method in public settings, or likely even for specific uses such as disinfection of masks.

Viruses such as SARS-Cov-2 attach very quickly and in a manner that nasal lavage would not be an effective method of infection prevention – virus can’t be washed off cells after it attaches. The virus does take some time to infect a cell (on the order of hours) and spread from cell to cell within a tissue (on the order of days), so it is hoped that therapeutics administered early in an infection might prevent or slow down disease.

Commercial antibody tests for persons with insurance are available at healthcare providers like hospitals and physician’s offices now. However, most of the tests have a very high false positive rate, and detectable antibody levels wane in some people after 3-4 months (immunity does not go away, but the antibodies circulating in the blood do). So neither a positive test nor a negative test is likely to be terribly accurate.

If you are worried you might have COVID-19, you can text COVID to 901-203-5526, answer the questions you receive, and follow the prompts. If you qualify for testing, someone will call you and make an appointment for testing at a UTHSC testing site.

You should call your doctor as a first measure, and not go out in public where you might make others ill. The procedure if you have the symptoms is to call your doctor, who, when appropriate, will either test you in the office or refer you to a community testing site. A complete list of free community testing sites can be found on the City of Memphis COVID-19 website.

Yes, you should be tested prior to return to work.

The most common test being used by is a molecular test to detect the genome of the virus (i.e., the RNA or nucleic acids). Typically respiratory specimens such as nasal swabs, sputum, and broncho-alveoalar (lung) lavage fluid are being tested – although there are blood tests in use for other coronaviruses such as MERS or SARS, these are not currently in use to assess for COVID-19. The test itself takes a few hours to do, but at present, it takes 24-48 hours to return a result because of the need for shipping, running multiple tests at once, and the general procedures of the laboratories involved. “Rapid” tests are also available now – these take between 15 minutes and a few hours depending on the processes involved in processing the test, but these methods are less accurate that the molecular tests. Some testing centers now use saliva or oral swabs, but this results in a somewhat less accurate result.

The false positive and false negative rates of the tests depend on the modality used. They are likely to vary minimally between commercial laboratories but may vary substantially when comparing commercial laboratories and private or hospital based laboratories. The sensitivity and specificity of the commercially available tests under laboratory conditions are essentially 100%. The “real-world” sensitivity and specificity are about 80-90% and 99.5%, respectively, for molecular tests (PCR) using a naso-pharyngeal swab; the sensitivity and thus the false negative rate are lower outside of laboratory conditions because of host issues (potential inhibitors in the nasal secretions, tests administered too early or too late in the infection course), technique issues (poorly trained personnel, patients reacting to the swab insertion with early withdrawal), and testing issues (failure to keep proper temperature during storage and transport, issues with the test protocol itself, etc.). Use of oral swabs or saliva decreases sensitivity by 10-20%, and rapid tests further decrease sensitivity by 10-20%. Specificity is also worse with rapid tests.

Yes, and many pediatric offices now have rapid “screening” tests for COVID-19.

Many respiratory viruses cause a spectrum of issues from upper respiratory tract symptoms (runny nose, congestion, sore throat, sneezing, coughing up mucus) to systemic symptoms (fever, chills, muscle aches, tiredness and malaise) to lower respiratory tract symptoms (cough, chest pain, shortness of breath). There is a large group of more than 200 viruses (including some human coronaviruses) that mainly cause the upper respiratory symptoms and are generally lumped together as the common cold. Some of these, like parainfluenza viruses and respiratory syncytial virus (RSV), may cause only colds in healthy adults but may cause pneumonia in babies and in the elderly. Influenza causes upper respiratory tract symptoms in most patients just like the common cold but can also cause systemic symptoms and pneumonia in many other persons including healthy adults. The novel coronavirus is interesting in that very few (only 2-3%) patients have any upper respiratory symptoms – fever, cough, and shortness of breath or chest pain are the cardinal symptoms without the typical common cold presentation that is usually seen simultaneously with influenza. COVID-19 can also present with a wide array of other symptoms, including sore throat, diarrhea, general fatigue, or loss of taste or smell. In summary:

• Common cold: upper respiratory tract symptoms (runny nose, congestion, sore throat, sneezing, coughing up mucus)
• Influenza: upper respiratory tract symptoms and systemic (fever, chills, muscle aches, tiredness and malaise) and lower respiratory tract symptoms (cough, chest pain, shortness of breath)
• COVID-19: systemic and lower respiratory tract symptoms, plus novel presentations such as loss of taste and smell.

Vaccines are very, very difficult to make. We still do not have a vaccine for the SARS coronavirus, which first appeared in 2003. Scientists have been researching and testing vaccines for Respiratory Syncytial Virus (RSV) since the 1960s without success. Part of this is just the complexity of our immune systems and the many ways that viruses have evolved to evade them – it is difficult to generate strong, protective immune responses to some viruses. Part of this is lack of funding and interest from the US Government. In the 2000s when our government became interested in developing vaccines against avian influenza strains, a great deal of money was poured into this problem. Scientists here in Memphis, at St. Jude Children’s Research Hospital, developed a system to make vaccines quickly and safely – a new clinical grade vaccine can be produced in weeks, tested for safety and its ability to generate immunity at sites around the globe within months, and can be available in quantity to the public in under a year. We do not have anything approaching this infrastructure for coronaviruses because the funding has simply not been available. The good news is that efforts to generate vaccines against the SARS coronavirus had been in early human testing, so those techniques have been adapted to the novel coronavirus. Several vaccines are under study right now, and some candidate vaccines may reach the approval stage in the next 6 months. For more information go here: 

Superoxides, cold plasma, ozone treatment, ultraviolet light, and other germicidal modalities have been tested against many viruses and do exhibit killing activity against many of them. Some of these are utilized in niche industries, such as in treatment of wastewater. They have not come into common use in healthcare or household settings mainly because of commercial reasons – there is little perceived need for new treatments, standard disinfectants work fine at low cost, there are cost barriers to market entry, and most of these treatments are not patentable. Ozone therapy in humans has been tested but is not at a stage where it could become an FDA approved therapy any time in the near future.

A UTHSC we are testing a number of drugs in our regional biocontainment laboratory (news story) that have activity against coronaviruses and are starting to work with other companies and individual scientists who have similar drugs. Because the path to bring a new drug to market is typically years, any potential cures that come out of this work are unlikely to impact the current pandemic. We and others have also been looking at existing drugs that cant be re-purposed to use in patients with COVID-19. One is called remdesivir – this is an anti-viral drug developed for use against HIV. Because it targets protease activity (clipping proteins important to the virus), it has activity against several viruses that require proteases in their life cycle, including HIV and the novel coronavirus. A second drug is called tocilizumab – this is a monoclonal antibody that is used in rheumatoid arthritis to treat inflammation. Cocktails of antibodies developed against the novel coronavirus have also been developed and are being studied in clinical trials led by UTHSC.

Allergies should not be an issue, but chronic bronchitis puts you at risk for poor outcomes if you were to be infected with the novel coronavirus. The only current advice is to avoid others who might be sick and wash your hands frequently and before touching your face. Once the virus becomes more widespread, it might make sense for elderly persons with chronic heart or respiratory disease to stay home as much as possible. To prepare for this possibility, make sure you have a good supply of any necessary medicines at home and any other essentials you might need. Also, I would try to make household plans for what you would do if a home caretaker were to become sick or for who would care for you were you to become ill.

The general recommendation at present is for persons who test positive and are isolated at home to stay inside until 10 days have passed and their symptoms have been gone for at least 24 hours. Persons who are under self-quarantine for a high risk exposure to a person who is infected are at lower risk and could go outside for exercise as long as they practice good social distancing – according to the CDC a low risk exposure (casual contact for less than 10 minutes) does not require self-quarantine. An infected person should not be riding elevators if they are on self-isolation – if they are required to (such as to visit a doctor’s office), then they should ride alone and always wear a mask. Public areas like elevators should be cleaned at least once a day.

If you had COVID-19 and 3-4 weeks have passed, you should have antibodies in your blood that will protect you from future infections. We can measure these antibodies with a blood sample. However, these detectable antibodies wane in some people after 3-4 months, so you may not be positive forever.

There are a couple of pieces to consider in this question. First, it takes a certain amount of time to make a vaccine (months to years) regardless of how much money you throw at it. Fortunately, we had already been working on coronavirus vaccines (against SARS) for 17 years when the COVID-19 pandemic started, so adapting candidate vaccines to SARS-CoV-2 could be accomplished in a matter of months and in some cases exactly this has been done. Next, the vaccine must be tested for safety and efficacy (will it cause immune responses and lead to protection from infection) in animal models before it can be used in humans, which takes months. Next, it has to be tested in humans for safety, to determine safe and effective doses, and then for immunogenicity or protection in humans (take months to years). This is the stage that we might be able to speed up by spending a lot of money. Next, you must develop versions of the vaccine under pristine conditions so they are pure, and you can be assured they are not contaminated with something that can be harmful to humans (weeks to months). Next, you must massively scale manufacturing up to make the 300 million+ doses we would need in the United States, to say nothing of the rest of the world (this would take years). Next, you must distribute the vaccine throughout the US and find a way to give it to everyone (months). So 18 months would be very, very rapid production, distribution, and utilization of a vaccine for COVID-19, and I think is an optimistic estimate. One or two vaccines might make it earlier than this timeline, but not many.

CDC provides general guidance for businesses and will update it periodically as the pandemic spreads. Among several recommendations, the most relevant advice is to actively encourage sick employees to stay home and send home any employees who develop respiratory symptoms (cough, shortness of breath). Emphasize good hand hygiene and provide soap and water and alcohol-based hand gels. Perform at least daily routine environmental cleaning of all surfaces in the workplace that are commonly touched, and employees may wish to wipe down commonly used surfaces (e.g., shared keyboards, coffee makers) before each use. Make sure everyone wears a mask at all times unless in a closed office that does not share an air supply with another office. If the neighboring firm has employees traveling to areas where there are outbreaks, you should probably ask that those persons don’t use your shared areas for 14 days after returning.

Yes, countries can impose quarantines on international travelers if they feel it is in the best interests of their people. If there is a chance you were exposed abroad, you could be quarantined in place, in a governmental facility, or banned from places like airports. The United States may also quarantine you upon return home in some circumstances.

Most deaths are in persons over 55 with chronic medical conditions. The CDC currently advises persons in this category to consult with their doctor prior to flying to have a more nuanced discussion of risk. At this point, many physicians are recommending avoiding travel to areas with active outbreaks to decrease risk of infection and possible complications. Since COVID-19 has become widespread in Memphis, the rationale for this is less powerful since you could be infected here as easily as away from home, and the decision will rest on factors like: does my insurance cover me in another country? Is there a risk that I will become stuck away from home due to flight cancellations, quarantines, or new travel restrictions? With many businesses and public events being cancelled, will the purpose of my trip still be valid (e.g., tourism, business meetings). Travel at present is a calculus based on tolerance for personal risk and the resources to handle potential delays and disruptions in travel.

Different strains of coronaviruses infect many different animal species, but these viruses do not often cross species barriers and infect other animals. The SARS coronavirus and this novel coronavirus were both derived from bat viruses originally and then infected an intermediate mammalian host – likely civet cats in the case of SARS. Speculation about the intermediate source of the novel coronavirus currently centers on pangolins, an armadillo-like animal that is traded extensively on the black market because its scales are used in traditional medicines. Pangolins in the Wuhan market where the first human cases occurred tested positive for a very closely related coronavirus strain. So, it is possible that this virus could further cross over into domestic pets or other animals, but it is very unlikely based on what we know at present. If it did infect cats or dogs (as has been speculated in a very small number of cases), they would be an unlikely source of further transmission. Humans are going to remain the dominant source of further spread of this virus.

N95 are often used by healthcare workers for protection from infectious agents. They are superior to surgical masks because they are more tightly fitted and are less likely to pass pathogens either around the mask or directly through. However, they require training and are not terribly effective in community settings. An N95 mask must be the right size and must fit tightly – in hospitals, healthcare workers are fitted and certified prior to wearing them. Facial hair may prevent a tight fit and make the mask ineffective. Masks do not cover the eyes, so infection by touching your eyes with contaminated hands can still occur, and if you contaminate the outside of the mask or contaminate your hands and reach under the mask to your mouth or nose you have lost any protection you had.

The best way to prevent infection is not to go near someone who is sick with the disease and wear a mask or face covering at all times when around others. If you are in areas with active disease outbreaks, then the best measures are to stay at least 6 feet away from anyone with symptoms, avoid close contact such as shaking hands, and practice frequent hand hygiene. The novel coronavirus can be killed on your hands or on other surfaces with normal varieties of soap, disinfectants, and alcohol solutions such as hand gels. Special disinfectants are not needed. Wearing a mask, frequently cleaning your hands and being very careful not to touch your eyes, nose, or mouth without cleaning your hands are the best prevention during this outbreak.

The novel coronavirus can be killed on your hands or on other surfaces with normal varieties of soap, disinfectants, and alcohol solutions such as hand gels. Frequently cleaning your hands and being very careful not to touch your eyes, nose, or mouth without cleaning your hands are the best prevention if you are in an outbreak. It is likely that scarves or other traditional garments that cover the face will have some impact on transmission of the virus, particularly in instances where large droplet spread is occurring person to persons. The materials used to make scarves and wraps will not stop penetration of these viruses if they are aerosolized.

Most patients with HIV infection in the United States are now treated with highly active antiretroviral medications and have a normally functioning immune system, so are unlikely to fare any worse against with COVID-19 than the general public. This may be more of a problem in sub-Saharan Africa, where treatment is not comprehensive. Patients on biologics that feature some immune-suppression may or may not have increased susceptibility to infection with the novel coronavirus or experience worse outcomes with COVID-19, but we don’t really have any data at this point. At present, the best recommendation of UTHSC experts is to take personal precautions as are recommended for the general public and discuss your ongoing therapy with your physician if you are concerned about specific issues such as travel to affected areas.

The best method for cleaning hands is normal soap and water, with about 20 seconds devoted to cleansing. Alcohol hand sanitizer is an acceptable alternative if soap and water are not available, but alcohol may take some time to kill the virus and without the mechanical action of scrubbing the hands and the water to wash the emulsified virus off, may not be quite as effective. Most standard disinfecting cleaners will kill the virus on surfaces (as will soap and water). Special disinfectants are not needed, so you do not need to look for claims of activity against this virus or others. The CDC has put forth guidance on how to blend a disinfectant solution from bleach — five tablespoons (1/3 cup) of bleach per gallon of water (and never mix bleach with ammonia or any other cleanser). All surfaces in common use should be wiped down daily if shared with persons who might unknowingly be infected, with additional spot cleaning in between for high use items and if concerned.

The elderly, persons with chronic respiratory or cardiac diseases, pregnant women, and immunocompromised persons are at high risk for poor outcomes from COVID-19. General guidance is that while the virus is in circulation, persons at risk should avoid going out in public places where they might have contact with infected persons. In the home, the same issues are present, but are reduced because it is a more limited set of exposures. Many living and caretaking arrangements can be very complex and require careful and precautionary planning. Persons in a household with or caring for persons at risk should take extra precautions themselves to avoid bringing the virus into the household. Plans should be made to ensure care of persons at risk should their primary caretaker or others in the household become sick. We may all want to work with our neighbors to develop these plans for the many unique situations that will arise during the pandemic.

Coronavirus can survive on surfaces for up to a few days under ideal conditions. The average length of survival is about 2-3 hours, with decreasing length of survival in warm conditions, low humidity, on smooth surfaces, and on metal, and longer survival in cold, moist conditions and on rough surfaces. So a mailed document is exceedingly unlikely to transmit virus in this scenario. A document dropped off in person by someone with an active illness could potentially transmit for some hours if they coughed on it, you touched it, and then you touched your face. So it would be possible, but still unlikely.

Most high contact sports have been canceled in the Mid-South out of an abundance of caution, but many team sports have resumed. The virus should be easily transmittable in contact sports such as basketball or football, but it much less likely in distance sports such as tennis, golf, pickleball, track, etc. Protocol is to keep typical social distancing (greater than 6 feet) at all times, avoid shaking hands or otherwise touching other individuals, and don’t share equipment. Presently in Tennessee and in Memphis there are no prohibitions on outdoor activities (or even well-spaced indoor activities), so long as common sense and social distancing are employed, and a mask is worn if there is the possibility of closer contact.

Welcome to our first Spanish language question! Persons who test positive for COVD-19 are asked to self-isolate themselves at home. This means not going out of the house, and trying to avoid contact with other persons in the household. Ideally, the infected person would be put in a separate room or part of the house away from everyone else. If you have been in close contact with your son (less than 6 feet for more than 10 minutes), such as at the dinner table, then you have likely been exposed. At present, the best advice would be to quarantine yourself by staying at home for up to 14 days. If you do need to go out wear a mask. And of course if you develop symptoms yourself, stay home but call your doctor for advice.

If you are both infected, then there is no reason to isolate from each other within the household. I would be cautious though assuming that you both have the same thing – if only one of you is tested and we assume that the second person with symptoms has the same infection, you might actually have two different viruses and expose the infected person to COVID-19.

Your husband should be taking every precaution to avoid persons who are ill, avoid touching surfaces that might be contaminated with virus without using some sort of barrier protection, wear a mask at all times, and avoid touching his face. It might be a good idea to wear gloves at work if he is in frequent contact with the public, or to use a handkerchief or other cloth to do things like open doors. He should utilize good hand hygiene at all times. If he becomes ill, he should get tested as soon as possible, and should stay away from you until the test has returned negative.

Persons who are infected will have no symptoms for the first 5 days on average (with a range of 2-14 days), and then will feel ill. We think that an infected person can transmit the virus for about 2 days before they feel ill and during the entire period when they are sick, although the odds of transmission drop over the course of the infection. We do not know how long after they feel better that a person could still transmit the disease – CDC is estimating 2-3 days, but it could be longer. Altogether, it is likely that a person is infectious for about a week overall.

We have not studied and compared different fabric materials for their protection from viruses such as these. Fabrics with a denser weave, multiple layers, and a tight fit to the face should be better than simple fabrics, but how much better is unclear.

High-dose systemic steroids are known to inhibit the body’s response to viral infections, in some cases leading to worse outcomes. Persons who are immunosuppressed in this manner should take every precaution to avoid COVID-19 infection. Inhaled or intranasal steroids are not thought to lead to worse viral infections, although there have not been any studies done yet with the actual SARS-CoV-2 virus, and there is some evidence that intravenous steroids can provide a benefit in sicker individuals who already have COVID-19. Albuterol and other inhaled treatments for asthma should be fine for normal use.

Yes, this is an acceptable strategy. The virus should not survive for days on a mask unless it is damp and in a cold environment. I keep several masks in my glove compartment and put one on when I go out, rotating them. If you are using cloth masks or use pocket squares or handkerchiefs to open doors and pick up items in public like I do, they can be thrown in the laundry and cleaned that way. The virus will not survive a normal wash/dry cycle on a cloth item.

This is a good question to ask a doctor, as it depends on several factors. If you are infected and get tested early on in your infection, you may test negative because the virus has not yet grown enough to be detected. In that case you would want to self-quarantine. If you are getting tested later after exposure (maybe 7-10 days), a negative test is a pretty good indicator that you didn’t get the virus. However, we may be more cautious in someone like a front-line healthcare worker than in someone who doesn’t work around people, so there are lots of permutations to consider.