Health experts warn of rebound effect after flu cases plummeted in 2020

Health experts are warning that the flu and other viruses besides COVID-19 could see a resurgence in the next year following a sharp decline due to social distancing and other safety restrictions amid the coronavirus pandemic. 

As people across the world take health precautions against COVID-19, other viruses, including influenza A, influenza B, parainfluenza, norovirus, respiratory syncytial virus (RSV) and human metapneumovirus, all appear to to be occurring at or near levels lower than previously measured, according to data compiled by The Washington Post. 

The same low rates have also been reported for respiratory bacteria that cause pertussis, better known as whooping cough, and pneumonia.

As of the week ending Jan. 2, the Centers for Disease Control and Prevention (CDC) reported that most U.S. states have shown minimal to low levels of influenza. 

In November, experts said 2020’s influenza transmission appeared to be one of the lowest in recorded history.

During the third week of December in 2019, the CDC’s network of clinical labs reported that 16.2 percent of the 29,578 samples tested were positive for influenza A. This rate was just 0.3 percent for the same week in 2020, according to the Post. 

The lower rates also came amid a record number of flu vaccine doses administered, with the CDC noting in early December that approximately 188 million had been distributed. This number was up to 192 million by the end of the year. 

However, experts told the Post that as COVID-19 vaccines become more widely distributed in the coming months and restrictions on activities start to relax, the lowered immunity people have to other pathogens from lack of exposure could have a rebound effect. 

“The best analogy is to a forest fire,” said Bryan Grenfell, an epidemiologist and population biologist at Princeton University. “For the fire to spread, it needs to have unburned wood. For epidemics to spread, they require people who haven’t previously been infected. So if people don’t get infected this year by these viruses, they likely will at some point later on.”

The Post noted that this effect already appears to be taking place in Australia, where in the last few months, as the coronavirus was virtually wiped out in the country, the number of flu cases among children ages 5 and younger began to climb. The flu was at historically low levels in the country at the beginning of May 2020. 

The possibility for a rebound comes as results last month from a phase one trial indicate that it may be possible to immunize people against all strains of influenza. 

Florian Krammer, co-author of the study at Mt. Sinai and a vaccine specialist, said the need for an annual flu vaccine could become obsolete if the vaccine does well in both the phase two and phase three trials, which are needed to get approval from the Food and Drug Administration. However, he noted that a lack of funding for research would likely stall the availability of the vaccine for a few years.