You can have COVID-19 without symptoms, but what about the flu?

Just because you’re not sneezing doesn’t mean you’re not contagious.
A collection of items used to ease flu symptoms: a glass of water, hot water bottle, allergy pills, a thermometer and a stethoscope
It's time to prepare for flu season. Kristine Wook

Just because we’re in a pandemic doesn’t mean that flu season is canceled. In fact, rising influenza cases are expected to correlate with spikes of coronavirus infections as the weather cools down. But it’s important not to wait until you start sneezing to take precautions to keep from spreading the flu to others—it’s possible, and even likely, that you can pass the flu to other people before ever showing symptoms.

To understand how the flu is transmitted before symptoms set in, it’s important to know the difference between pre-symptomatic and asymptomatic transmission. In pre-symptomatic influenza cases, a person has been infected with the flu but has yet to develop symptoms. In general, it takes about two to three days after infection for symptoms to set in, according to Seema Lakdawala, an assistant professor at the University of Pittsburgh who researches influenza viruses. During that time, the infected person is shedding virus particles whenever they exhale, and other people can catch the flu from those particles.

“When somebody gets infected, it takes a while for them to develop symptoms associated with the viral infection,” says Lakdwala. “Before they have a fever or a cough or feel lethargic, the virus has already affected them and they are already shedding large amounts of virus.”

In contrast, an asymptomatic person will never develop symptoms. However, they will still develop antibodies which will help their immune system to deter future infections. Researchers are not entirely sure whether asymptomatic flu cases can transmit the virus. “You can argue there is some evidence that you can’t,” says Lakdawala, but studies have been mixed. Asymptomatic individuals do shed viral particles, it’s just not clear how often they’d shed enough to infect another person. And it’s further complicated by the fact that mild symptoms, like sneezing, might not make you feel particularly sick even though you do, in fact, have the flu. About one in three cases of the flu are asymptomatic, but it’s important to remember that the same strain that didn’t make you all that sick could be serious for anyone elderly or with a compromised immune system.

So yes, you can pass on the flu before you develop symptoms—but if you never develop symptoms, evidence suggests it’s unlikely you were ever contagious, even if you have antibodies. (It’s worth noting that the Centers for Disease Control and Prevention currently reports that the novel coronavirus behaves in a similar way, though people with COVID-19 can be pre-symptomatic for far longer than people with the flu.)

People do have a natural immune response specific to one strain of the flu, and recent research indicates that your first flu infection, which usually occurs before the age of five, determines the strain you are most equipped to fight off. But the influenza virus mutates rapidly, so the vaccine has to change every year to try to give the population a reasonable level of immunity. People may respond differently to the flu vaccine depending on which strain of flu they have antibodies for, but no matter what, getting your flu shot each year lowers your chances of catching flu and passing it on to other people.

That’s why the best way to help yourself and society at large is to get your annual flu vaccine.

“Immunity is a huge barrier for transmission,” says Lakdawala. “The way that viruses will stop transmitting in our population is when there is immunity, either through natural infection or through vaccination.” Even though the flu vaccine doesn’t give 100 percent protection, lowering the risk does give us some degree of protection as a community.

On top of getting the flu shot, there are other methods of preventing the spread of influenza, and they’re the same methods the world has been using to deter the spread of the coronavirus: social distancing, wearing masks, and washing hands regularly. Though flu season is just beginning in the US, COVID-19 precautions have already been linked to a drastic decrease in influenza cases in Australia, where peak flu season is during the US summer months.

“By staying home when you are sick, wearing a mask when you are sick, keeping a distance from people and not trying to power through when you don’t feel good by going to school or going to work, we have seen the burden of influenza infections go down dramatically,” says Lakdawala. “That, to me, is really representative of what we can do if we are really serious about limiting influenza burden.”