With No Flu Season, Next Year's Vaccine Prep is Tricky

Brian Owens

March 03, 2021

Experts predicted that the 2019/20 flu season in the Northern Hemisphere would be a bad one. And then it wasn't.

The COVID-19 pandemic hit, international travel ground to a halt, physical distancing and mask wearing became more common, and the flu went away.

Rachel Baker, PhD, who studies public health and infectious disease at Princeton University's High Meadows Environmental Institute, said that in mid-March 2020, "there was a clear drop-off right when lockdowns were announced." This was particularly evident in tropical places like Hawaii, which usually has persistent cases throughout the year rather than seasonal peaks alternating with periods in which cases drop to zero.

Part of the sudden decrease might be the result of health authorities in some areas focusing all their attention on COVID-19 and essentially not testing for influenza, said John McCauley, PhD, director of the Worldwide Influenza Centre at the Francis Crick Institute in London, one of the six influenza surveillance centers in the World Health Organization's Global Influenza Surveillance and Response System.

More Americans opted to get a flu shot during the 2019/20 season than during the year before, according to the Centers for Disease Control and Prevention. The rate increased by 3.1 percentage points, with 48% of adults receiving the vaccine. Vaccination rates for children, however, remained the same.

Clearly, health measures designed to slow the transmission of COVID-19 can be just as effective at reducing the spread of influenza and other respiratory infections.

Baker and her colleagues estimate that COVID-19 precautions led to a 20% drop in the transmission of respiratory syncytial virus (RSV) across the United States last spring, and likely had a similar effect on influenza. And this might be an underestimate because another study from Hong Kong showed a 44% decline in flu transmission after the introduction of public health measures.

"This is one of the clear signals that these interventions work," said Baker. "They are really efficient at stopping the spread of these viruses."

"Strikingly Different" This Year

The situation is "strikingly different" this year, with no flu in the Northern Hemisphere during the winter of 2020/21, Baker said.

There was "no flu season to speak of" in the Southern Hemisphere either, added Ian Barr, PhD, deputy director of the WHO Collaborating Centre for Reference and Research on Influenza at the Peter Doherty Institute for Infection and Immunity in Melbourne, Australia.

This made things difficult for experts like Barr and McCauley, who contribute to the global planning of which strains of influenza should be included in the seasonal flu vaccines. A decision becomes trickier when there is a dearth of information about what strains are circulating in the population.

And it's not like they weren't looking.

"We've scoured the planet to find where the viruses are," Barr said. Overall, more samples were tested this year than last, but there was very little flu to find.

There were minor outbreaks in Bangladesh, India, in Southeast Asia, in Western Africa, and in Europe, but that was about it. That left the WHO team with a dilemma as they developed the 2021/22 vaccine for the Northern Hemisphere: Should they keep the strains in the vaccine the same — assuming the ones in circulation before the pandemic will come back once it is over — or change it to reflect what they were seeing in small numbers around the world?

The team would have preferred to have more data to guide the difficult decision, McCauley explained. But in the end, they opted to stay the course. Three of the four strains in the 2021/22 vaccine will remain the same as last year's Southern Hemisphere version. They changed just one — an H3 influenza A strain — to what was found in circulation.

What happens next is unclear.

Next Year's Outbreaks

Modeling suggests that as population immunity wanes over the course of the "missing season," big outbreaks could become more likely after life goes back to normal and public health measures implemented during the pandemic are eased, said Baker. She predicts substantial outbreaks of RSV in future years, with a likely peak in the winter of 2021/22.

For flu, the picture is complicated by the transmissibility and evolutionary dynamics of the circulating strains and the effect of vaccines, but a similar outcome is possible.

However, with fewer infections, the influenza viruses haven't had as many opportunities to mutate and evolve. That means the strains that pop up in the future will likely be similar to the ones circulating before the pandemic, so the vaccines will remain effective.

And the viruses will be starting from a low baseline, said McCauley, so they might not have time to spread to large numbers of people during the seasonal window.

It will likely take several years before flu numbers return to normal levels, Barr said.

Still, "we have to plan for a worse season," McCauley said. Even if a worst-case scenario never comes to pass, effective vaccines that are both available and widely used will help guard against a big influenza outbreak. This is especially important at a time when the COVID-19 pandemic is not completely behind us because "the two together would be very difficult to manage," he noted.

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