Contagious Periods for Colds, Flu, COVID-19 and RSV: Factors and Precautions - Everyday Health

How Long Is the Flu Contagious?

Most human flu illnesses are caused by influenza A and B viruses, which can infect the nose, throat, and sometimes the lungs, according to the CDC.

Symptoms of the flu can range from mild to severe, and include:

• Fever
• Chills
• Cough
• Sore throat
• Runny or stuffy nose
• Muscle aches
• Headaches
• Fatigue
• Vomiting and diarrhea (though the GI symptoms are more common in children)

Flu is mostly spread through the air by tiny droplets generated when infected people cough, sneeze, or talk, which can then go on to land in the mouths or noses of others. It can also be spread by contact with objects — if a person touches something with the flu virus on it and then touches their nose or mouth — though that's less common.

As with a cold, you can shed the flu virus and infect other people up to 24 hours prior to onset of symptoms, says Winslow. "You're most contagious the first day or two after the onset of symptoms," he says.

By day three or four, you may still have a lot of symptoms as a result of your immune system being activated, but the amount of virus that you're shedding is lower, he says. "Even though it's less likely to spread by that point, you are still potentially contagious. To be safe, I would recommend that people wear a mask pretty much the whole time they have upper respiratory symptoms," says Winslow.

The flu shot can reduce your chances of getting the flu. If you do get the flu even after getting immunized, it's less likely that you'll become severely ill.

There isn't enough evidence to show that already being vaccinated against the flu influences how long you'll be contagious if you do still get the virus, says Winslow.

But a medication such as Tamiflu taken within the first 48 hours after onset of symptoms probably does make you less contagious, says Winslow. "It definitely reduces the duration of symptoms and also very likely has at least a modest effect on reducing viral shedding because it is an antiviral," he says.

How Long Is RSV Contagious?

RSV is a highly contagious virus that causes inflammation of the respiratory tract and usually causes mild, cold-like symptoms, according to Yale Medicine. It can be hard to tell the difference between a cold and RSV, though the latter usually involves more mucus production (meaning you go through a lot of tissues).

People infected with RSV usually show symptoms within four to six days after infection, per the CDC.

Symptoms of RSV infection usually include runny nose, decrease in appetite, coughing, sneezing, fever, and wheezing, according to the CDC.

RSV is spread through droplets released into the air when an infected person coughs or sneezes, direct contact (like kissing someone with RSV), or from touching a contaminated surface (like a doorknob) and then touching your mouth, nose or eyes, according to Yale Medicine.

Again, you can spread RSV a day or two before your symptoms show up, and then typically for about three to eight days after your exposure, per the CDC. After the first few days of symptoms, the amount of virus you shed decreases dramatically — unless your immune system is compromised, says Winslow.

Some infants and adults with weakened immune systems can't eliminate the virus as well as healthy adults, and remain contagious for as long as four weeks, according to the CDC.

The RSV vaccine is relatively new, and it isn't known whether it can affect how long you are contagious, says Winslow.

How Long Is COVID-19 Contagious?

At this point, COVID-19 is very contagious, says Winslow. "The way the virus has evolved is that it's tended to become more contagious and cause less disease, which is a common strategy that most pathogens take in terms of their evolution," he explains.

COVID-19 symptoms can range from mild to severe, and can include:

• Fever
• Chills
• Cough
• Shortness of breath
• Fatigue
• Muscle or body aches
• Headache
• Loss of taste or smell
• Sore throat
• Congestion or runny nose
• Nausea
• Vomiting and diarrhea

The virus spreads via droplets and very small particles exhaled by people with COVID-19. Others can then catch the virus if these particles are breathed in or land on people's eyes, noses, or mouth. More rarely, virus particles can land on surfaces and be spread that way.

"Research has confirmed that you can be contagious up to 24 hours prior to onset of symptoms and then you are most contagious one to two days after the onset of symptoms," says Winslow.

The severity of your illness impacts how long you are contagious. People who are infected but don't have symptoms or people with mild COVID-19 should isolate through at least day 5 (day 0 is the day symptoms appeared or the date you tested positive), per the CDC. The agency recommends continuing masking through day 10, or once you test negative.

People with moderate or severe COVID-19 should isolate through at least day 10. Those with severe COVID-19 can remain infectious beyond 10 days and may need to extend isolation for up to 20 days.

Moderately or severely immunocompromised people may be contagious longer and should isolate through at least day 20, per the agency. Use of serial testing and consultation with an infectious-disease specialist is recommended to determine when to end isolation.

If you are vaccinated for COVID-19 but get the infection anyway, you can still spread the virus, but may be less contagious because you shed the virus for a shorter period, according to a Harvard study.

It has been shown that symptom severity generally correlates with the viral load, and so it makes sense that if you have some preexisting immunity from the vaccine, you're less likely to be shedding the virus, says Winslow.

Antiviral drugs (such as Paxlovid) taken shortly after symptoms start may reduce viral shedding, but the recommendations on masking and isolating should still be followed, he says.

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