What to Expect When Covid-19 and the Flu Season Collide

In the first weeks of 2020, as Americans were in the midst of flu season, Australia saw an unusual uptick in influenza cases in the northern part of country, close to the equator. Occurring months before the perennial viral infection typically starts to peak in most of the Southern Hemisphere, Aussie public health officials saw the spike as a warning sign of a harsh flu season ahead.

Then, in March, as the Covid-19 pandemic took hold, the Australian government banned all non-residents from entering the country in an attempt to curb infection from outside visitors. The public was instructed to wear masks, stay physically distant from others and practice good personal hygiene.

Almost immediately, influenza cases fell. In March, nearly 6,000 laboratory-confirmed cases of influenza in Australia were reported, according to the National Department of Health. By April, the number had dropped to 308 cases. The previous year, April saw more than 18,000 influenza cases.

“We were wondering if we were going to see an early start on influenza,” says Kanta Subbarao, director of the World Health Organization Collaborating Center for Reference and Research on Influenza in Melbourne. “Then came along Covid-19 and it just fell off a cliff.”

Australia isn’t alone. Countries across the Southern Hemisphere experienced remarkably mild flu seasons. Chile has only recorded 247 influenza cases this season, compared to nearly 21,000 in 2019. Argentina reported 412 cases, down from 6,477 the year before. The National Institute for Communicable Diseases in South Africa reported only one case of the illness.

As countries in the Northern Hemisphere enter a new flu season, experts are looking to patterns from Southern Hemisphere nations as a source of cautious optimism.

Subbarao’s center in Melbourne is part of the World Health Organization’s Global Influenza Surveillance and Response System (GISRS), an international effort started in 1952 to monitor influenza and other respiratory pathogens. GISRS includes more than 140 national influenza centers, six collaborating centers, and various laboratories for vaccine development and virus surveillance.

“It isn’t for lack of looking,” she says. “People have been testing for influenza and we just aren’t seeing it.”

Lessons from the Southern Hemisphere

After Covid-19 had begun to spread globally in March, countries including Australia, Chile, Argentina and New Zealand barred international travel and shut down many businesses and schools to encourage physical distancing. The Southern Hemisphere countries with the most dramatic drops in flu cases this year enforced measures like self-quarantining and mask wearing early on in the pandemic.

By contrast, the U.S. has not closed its borders or mandated a quarantine period for incoming foreign travelers, and public officials now face greater pressure from people eager to reopen businesses and schools as the pandemic reaches its seventh month.

Compared to countries in the Southern Hemisphere that experienced mild flu seasons, the U.S. lacks a unified national response, opting instead to leave public health decisions to local or state governments. However, public health officials in the U.S. benefit from months of Covid-19 and flu research by scientists around the world.

“I think it’s okay to be hopeful that we will experience what the Southern Hemisphere did and have a mild flu season as we continue to work on our Covid-19 pandemic, but we still need to be prepared,” says Benjamin Singer, a pulmonary and critical care medicine specialist at Northwestern Memorial Hospital in Illinois.

The U.S. Centers for Disease Control and other top public health officials recommend people wear face masks, self-isolate when experiencing flu-like symptoms and avoid large social gatherings to limit the transmission of Covid-19. Because the influenza virus and SARS-CoV-2, the virus responsible for Covid-19, spread similarly—through respiratory droplets—these measures can be effective at quelling influenza, too.

To prevent a larger flu outbreak, however, these measures must be coupled with extensive testing for both the flu and Covid-19, as well as widespread flu vaccination and treatment, Singer says.

What’s the Risk of Coinfection?

Doctors are still unsure about the health effects of becoming ill with both viruses. And while a severe flu season could dangerously burden hospitals already dealing with many Covid-19 patients, it is unlikely that many people will experience coinfection.

Wuhan, China, experienced its flu season at the same time as its first major coronavirus outbreak. In January, a small study of 99 Covid-19 patients at a major hospital in Wuhan found no co-infections of influenza. Later studies in the U.S. also demonstrate low rates of co-infection. A study of 1,996 Covid-19 patients in New York City in April found that only one patient was coinfected with influenza. Scientists in Chicago reported in July that fewer than 1 percent of Covid-19 positive patients with flu-like symptoms were also infected with the influenza A virus.

Still, the possibility of co-infection cannot be ruled out. Gaining an exact diagnosis is important, Singer says, because scientists recommend different treatments for each virus. For example, the drug corticosteroid dexamethasone has been found to be harmful when used as an influenza treatment. But the same drug may effectively decrease the mortality rate of seriously ill Covid-19 patients also receiving respiratory support, according to a preliminary study.

Improving access to testing for both Covid-19 and influenza is also important for making sure the viruses don’t further spread. The CDC doesn’t currently recommend self-isolation for influenza, at least to the extent that is recommended for Covid-19.

“If flu does become severe this year—and I hope it doesn’t—we may see a change in the recommendation where people with the flu isolate in the same type of way that we ask people with Covid-19 to,” Singer says.

The Importance of the Flu Vaccine

Perhaps the most critical tool for public health in coming months will be the influenza vaccine.

The CDC currently recommends everyone older than six months receive a flu shot, with few exceptions. Last flu season, however, only 62 percent of children younger than 18 years old and 45 percent of adults received the flu vaccine, according to the CDC.

Edward Belongia, director for the Center for Clinical Epidemiology and Population Health at the Marshfield Clinic in Wisconsin, fears that mixed messages from public health officials and concerns about entering clinics or pharmacies during the Covid-19 pandemic could dissuade people from getting the shot even further.

“The flu vaccine has been around for many decades and it’s safety profile is very well known,” he says. The effectiveness of a vaccine dwindles over the course of a year, Belongia says. But this year, vaccination is especially important to protect against several new flu strains.

Each year, scientists determine the new vaccine composition in September for the Southern Hemisphere flu season and in February for the Northern Hemisphere. New influenza vaccines are comprised of antigens—substances that cause the immune system to produce antibodies—to protect against the most prominent flu strains known to be circulating at the time of development.

Most flu vaccines protect against a combination of four strains of the virus that circulate most widely each year. These strains mutate and change over time, so scientists swap out the antigens accordingly. Typically, the formula only includes one or two new antigens. This year, however, the vaccine protects against three strains expected to arise during flu season. Because most people’s immune systems haven’t recently been exposed to these three strains, the vaccine is needed to have a successful immune response.

“There’s a real benefit to an even 50 percent effective vaccine because it will reduce your risk both of getting the virus and of being hospitalized with flu,” Belongia says.

In recent flu seasons, hospitals were overburdened when H3N2—a particularly deadly strain of the flu—circulated. In the 2017-2018 season, for example, the U.S. recorded more than 30,000 influenza-related hospitalizations, according to the CDC. There’s no way to know for sure which strains will dominate this year, but if an H3N2 resurgence occurs, coupled with Covid-19, the strain on medical resources could be overwhelming.

“I don’t think hospitals are prepared for that,” Belongia says. “That would create some very difficult triage situations in terms of prioritizing care for people and limited access to [intensive care unit] beds or hospital beds.”

In an editorial he co-authored in Science, Belongia suggests especially vulnerable groups receive “enhanced [flu] vaccines.” Some of these contain higher doses of antigens or fewer mutations than the standard-dose vaccine, making them more effective for high-risk adults.

Elderly people aged 65 years or older account for most hospitalizations and deaths for both the flu and Covid-19, according to the CDC. In a study from June, which has not yet undergone peer-review, a group of researchers from Johns Hopkins University and Weill Medical College of Cornell University compared county-level public health data across the country and found that the influenza vaccine can decrease Covid-19 mortality rates for this demographic.

Despite its advantages, a moderate flu season this year poses a challenge for future preparedness. Scientists developing a flu vaccine for the Southern Hemisphere’s next influenza season have far less information about which strains are circulating, and which strains have died off. If they miss a new strain and it isn’t included in the next vaccine, even those who receive the vaccine could be left vulnerable.

“We have a big challenge now because we have very little information,” Subbarao says. “Once Covid-19 sort of settles down, next winter if we have a flu season, we actually don’t know whether all four strains will reemerge or whether one may have died out in this interim.”

For now, the influenza vaccine offers a tried-and-true method for flu prevention. As scientists continue to formulate an effective Covid-19 vaccine, encouraging the use of existing influenza vaccines and treatments is of utmost importance.

“We really have to try and minimize the risk of a serious flu epidemic by using the tools we have,” Subbarao says.