Symptoms, which most school and workplace policies rely on, are a surprisingly bad way to estimate whether you might be contagious. In January 2020, a loved one was hospitalized for RSV, and I started researching how long I should stay home if I had it, too. The CDC says people with RSV can be contagious for three to eight days. But symptoms can persist for weeks while some people may only have minor symptoms, which makes the agency’s “stay home when sick” advice inscrutable. (Some babies and people with weakened immune systems can spread the virus for up to a month, even if symptoms fade, researchers say.) In general, the sicker you are with RSV, the more contagious, it seems. But RSV only sometimes presents with fever. Fevers usually last two to four days among very young kids, while adults are much less likely to have a fever—making symptom-based recommendations difficult.
Similarly, the flu is contagious for about one day before symptoms develop, if they ever do, and about five to seven days after that, according to the CDC. Usually, viral load peaks on the second day or so of symptoms—which, if you get a fever, is often when the fever is spiking as well. But only two-thirds of people with the flu develop any symptoms at all. And there are marked differences between flu A and flu B; only 7.5 percent of flu B patients have a fever, compared to about 40 percent of flu A patients, and viral load and symptoms track much more closely with flu A than with flu B.
Crucially, Covid symptoms do not seem to correspond with contagiousness. “We knew surprisingly early on in the pandemic that asymptomatic individuals can shed virus and can transmit the virus to others,” said Sorrell. With omicron, only 22 percent of vaccinated people had a fever, and it was even less common among those who had a booster; even so, 80 percent of patients still tested positive after five days, one study found. Some 40 percent of kids are still contagious after symptoms resolve; one-quarter are contagious after seven days, and 10 percent are still contagious after 10 days.
