And once people began mingling more during the next flu seasons, widespread use of masks blocked influenza’s chances of infecting large numbers of people.
But that could change this flu season, as mask mandates have disappeared and more people are interacting in close quarters in school, workplaces, sports events, public transport, and more. Health experts are warning that flu cases could rise again this winter, and that the combination of influenza and COVID-19 together could pose a real public-health threat that sends more people to the hospital and in need of intensive care. Already, the flu season in the southern hemisphere—which runs from April to October and serves as a harbinger of what’s to come for the U.S.—has been severe, with cases in Australia three times higher than average compared to the past five years. That could mean influenza will sweep through North America and Europe with equally aggressive force this winter, alongside rising cases of COVID-19.
That opens the possibility that people could get the two infections at the same time—which experts believe could be both unpleasant and dangerous. “Are two viruses that cause huge inflammatory responses together going to make that response worse? Theoretically, yes,” says Dr. Khalilah Gates, a pulmonary critical care physician at Northwestern University.
Gates and others stress that there aren’t extensive data yet to be sure exactly what will happen when people are infected with both influenza and SARS-CoV-2. But the limited early data—some from people, but mostly from animals—are not encouraging. Already, doctors know that people who get both the flu and a cold at the same time tend to be sicker than those who are only infected with one virus. The same could be true when flu and COVID-19 combine; classic symptoms, including fever, chills, fatigue, and coughing, could become more intense for some people. In one 2021 study on COVID-19 co-infections, including 17 people who tested positive for both influenza and COVID-19 at King Fahad Hospital in Medina, Saudi Arabia, their rates of hospitalization and death were higher than those for people infected with COVID-19 a type of bacteria that can cause respiratory tract infections.
In the largest study so far looking at co-infection of the two viruses, published in April, researchers at the University of Edinburgh reported similar trends. Dr. J. Kenneth Baillie, professor of experimental medicine at the university, and his colleagues analyzed the health records of more than 212,000 people admitted to hospitals in the U.K. for COVID-19, who were also tested for other infections. People infected with influenza and SARS-CoV-2 were four times as likely to need mechanical ventilation, and twice as likely to die, compared to people who just had COVID-19.