How does flu spread compared with Covid? What to know as flu cases surge

Covid-19 has given us all a crash course in viral transmission and prevention. We know far more than we ever wanted to about how the virus that causes Covid spreads, how long it can incubate in the body and what kind of masks are most effective. That new awareness has made some of us ask similar questions about other viruses now that respiratory illnesses like flu and RSV are surging.

So how much do we really know about how flu spreads and how it compares with the virus that causes Covid? Here’s what scientists say.

How flu spreads

One of the big questions at the beginning of the Covid pandemic was how the virus transmits. Was it mainly through contact with contaminated surfaces? Was it droplets spreading when you’re close to an infected person? Or was transmission largely through aerosols, smaller particles that can be emitted and inhaled through talking and breathing, and which can linger in the air even after a contagious person has left?

We eventually learned that tiny aerosol particles are a major way Covid spreads, which is why indoor spaces are so much riskier than outdoor ones—because the particles fill up and linger in the air.

What about flu?

Most scientists agree that influenza is transmitted most commonly through the air, but there is disagreement about whether the main vehicle is aerosols or droplets.

Some scientists who study aerosol particles say flu mainly transmits through these tiny particles rather than through the larger droplets. That would mean you can get infected just by being in the same room with a contagious person—even far away—rather than having them sneeze on you or emit droplets while talking in proximity to them.

Studies show that aerosols containing influenza virus are infectious for more than an hour, says John Volckens, an environmental health professor at Colorado State University in Fort Collins, Colo., who studies aerosols.

Some infectious-disease doctors and the U.S. Centers for Disease Control and Prevention say that influenza is spread most often through droplets emitted from sneezing, coughing or talking, which requires closer contact with an infected person, usually within 6 feet.

“The closer you are, the easier it is to get flu," says Peter Chin-Hong, an infectious-disease specialist at the University of California, San Francisco.

Some studies suggest that most flu transmission is largely through short ranges, notes Dr. Chin-Hong. But a 2013 study in Nature Communications found that aerosol spread might account for about half of all household influenza transmissions, he said.

The CDC guidelines say that surface transmission of flu—by touching a contaminated surface such as a doorknob or table—is less common but possible. It is still a good idea to wash hands frequently, doctors say.

How contagious is flu?

Covid-19 is more contagious than influenza, doctors say. One reason is that most people have had flu multiple times and many have gotten multiple flu shots over the years.

The most common calculation of a virus’s infectiousness is a measure called the R0 (pronounced “R naught"). This metric estimates how many people one contagious person will infect on average. The R0 of influenza is between one and two. R0 data for Covid-19 isn’t definitive, especially as the virus continues to mutate, but studies indicate the number for many Covid strains is higher than for flu.

When are you contagious with flu?

The CDC and many doctors say you are likely contagious with flu a day before you develop symptoms, which can include a fever, congestion, cough, sore throat, headache, body aches and fatigue. Doctors also say it is possible to be infected with influenza and never develop symptoms, and that asymptomatic people can still transmit the virus to others.

Up to half of flu infections may be asymptomatic, says Dr. Chin-Hong, but people with such infections probably transmit less efficiently. A 2021 study in the Lancet Global Health found that asymptomatic individuals transmitted the flu to only about 6% of household contacts.

The incubation period for influenza—the time between when you get exposed to a virus and when you develop symptoms—is about one to two days, says Seema Lakdawala, an associate professor in the department of microbiology and immunology at Emory University who studies flu transmission.

Like people with Covid-19, those with influenza start shedding virus before they develop symptoms and can be contagious before they know they are sick, says Dr. Lakdawala. People with influenza are most infectious early on in their illness, in the first two to three days after being infected, she says.

If you’re older, have a chronic disease or are immunocompromised you may shed virus for longer.

Preventing the spread of flu

Covid-19 precautions also work against flu. Pandemic measures largely quashed the flu around the world for two years. Many of us aren’t used to taking those same precautions for flu. Doctors say we should reconsider—but we may not have to be as rigid about it.

Studies show that influenza can spread easily within a household. If someone in your home tests positive for flu, have them isolate as best as possible, recommends Kristin Englund, an infectious-disease physician at Cleveland Clinic. Eating separately or having them wear a high-qualitymask helps if strict isolation isn’t possible. If they are unable to wear a mask, others in the household should do so to lower risk. And when you’re in the same space, improve ventilation by opening the windows or running a HEPA air purifier.

Dr. Englund says schools and offices should re-evaluate their guidelines for returning after a flu infection. “It’s safest to wear a mask as long as somebody is symptomatic," she says, as congestion and coughs can linger for days.

Masking in indoor spaces and avoiding large crowds in poorly ventilated indoor spaces are other smart precautions to take to combat flu during flu season, says Dr. Lakdawala. Most important, stay home if you have symptoms.

“Be thoughtful," says Dr. Lakdawala. “We don’t need to take all the precautions that were maybe a little onerous during the Covid-19 pandemic, but we can still take some measures to help reduce risk in our communities."