How do you know if you are still contagious after an illness? A doctor explains

By Katia Hetter, CNN

(CNN) — It’s fall in the Northern Hemisphere, and the weather is getting cooler. Many people have cold-like symptoms, and some may have tested positive for Covid-19, influenza or the respiratory syncytial virus (RSV).

With winter on the way and viral infections increasing, a lot of people may wonder how long they will be contagious after infection and how long they should take precautions and avoid contact with others.

How can someone know if they are still contagious? If so, what’s the average length of time they could transmit viruses such as the coronavirus, influenza, RSV or the common cold to others? And what precautions should an infected person take at school, work and at home?

To guide us through these questions, I spoke with our CNN Wellness medical expert, Dr. Leana Wen. Wen is an emergency physician and professor of health policy and management at the George Washington University Milken Institute School of Public Health. She previously served as Baltimore’s health commissioner.

CNN: Is there an easy way to tell if someone is still contagious after having a viral infection?

Dr. Leana Wen: Not really. For many viruses, an infected individual is contagious even before they start exhibiting symptoms.

Also, while there are some tests that can tell you whether you have the virus, they usually aren’t used to clear someone after infection. The exception is Covid-19. According to the US Centers for Disease Control and Prevention, someone whose symptoms are improving and who has had two negative home coronavirus tests spaced 48 hours apart can end isolation.

One more complicating factor here is that many people may continue to have lingering symptoms well after their infectious period ends. It’s common to have a cough, for example, that could last for weeks or even months after a respiratory illness. Such symptoms indicate ongoing airway inflammation but do not reflect ongoing infection or risk to others.

CNN: What’s the average length of time someone could transmit the virus to others if they are infected with common viruses like the coronavirus, influenza, RSV or the common cold?

Wen: Let’s go through each of these. The virus that causes Covid-19, SARS-CoV-2, can be spread by asymptomatic individuals who are infected but don’t exhibit any of the common symptoms such as cough, fever, runny nose and sore throat. An infected person who has not yet developed symptoms—someone who is presymptomatic—may also be contagious.

Scientists estimate that the period of maximal contagiousness for SARS-CoV-2 is in the 48 hours prior to developing symptoms and then the first five days after. The CDC recommends that a person infected with Covid-19 should isolate for five days and then wear a high-quality, well-fitting mask when around people for the next five days, unless they test negative with two consecutive tests, as I mentioned above.

Asymptomatic transmission of influenza is probably less common. While it’s possible that you could spread the flu to others before you develop symptoms, the period of maximal contagiousness is thought to be three to four days after the illness begins. The CDC advises that otherwise healthy adults may be able to infect others up to five to seven days after becoming sick, and some people, like those with weakened immune systems, might be able to infect others for an even longer period of time.

RSV has a potentially longer period of contagiousness. Those infected with RSV, which is a very common respiratory infection believed to infect nearly every child before they turn 2, are usually contagious for three to eight days after they develop symptoms, though presymptomatic transmission is possible too. According to the CDC, some infants and people with immunocompromise could keep spreading the virus for as long as four weeks after they stop having symptoms.

As for the common cold, there are over 200 viruses that can cause it. The period of infectiousness depends on the virus and on the health of the individual infected, but generally follows a similar pattern to the other viruses discussed. In short, there could be a time before showing symptoms that the person may be contagious, and usually the period of maximal infectiousness is in the three to five days after symptoms begin. However, they may be able to spread the virus for as long as two weeks after infection.

CNN: With such long periods of potential infectiousness, what precautions should people with cold-like symptoms take at school and workplaces?

Wen: Schools and workplaces have long recognized that it’s unrealistic to ask that people stay home until the entire period during which they could be infectious passes. Even if they had policies asking that people stay out of school or work until, say, two or even four weeks after symptoms resolve, there will still be viral spread because of asymptomatic and presymptomatic transmission.

Also, given that children go through an average of six bouts of viral respiratory infections a year, and adults two to four, this kind of policy would result a lot of absenteeism, which has many effects on children’s learning and development, not to mention impacts on workplaces and the economy.

Many schools have policies that try to balance the importance of keeping kids in school with limiting in-person presence during times of maximal infectiousness. For instance, they may ask that parents keep kids home if they have a fever and that there is at least a 24-hour period of being fever-free without the need for fever-reducing medicines before kids return. Sometimes, they may require a doctor’s note if a child continues to have noticeable symptoms, like coughing.

Workplaces may have these or other protocols. If you are unsure, ask your human resources department. It’s generally a good idea to stay home if you have a fever and if you first start having active, new symptoms. If you are required to return to work, be sure to wear a well-fitting, high-quality mask in public or shared places to avoid infecting others.

CNN: What should people do to prevent infecting those they live with?

Wen: This is tricky. Parents of young children know that they often get sick when their kids do because it’s so hard to avoid infection when you are in close contact.

The key is to identify who in the household is particularly vulnerable to severe outcomes if they were to contract these viruses. If everyone is generally healthy, many families will decide that they won’t take extraordinary steps to isolate someone who has cold-like symptoms. On the other hand, if someone is elderly with serious medical conditions, it would make sense to separate the person who is infected from that individual. Those two people should not dine together or be in the same room for at least a week after symptoms begin.

These viral infections are very difficult to avoid, but I do want to emphasize that preventive methods can and do work. Wash your hands or use hand sanitizer often. And be sure to stay up-to-date on your vaccines, including the updated Covid-19 vaccine, the flu shot and, for those 60 and above, the new RSV vaccine. These vaccines, crucially, reduce the risk of severe illness, and they decrease your chance of getting infected with those specific viruses.

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