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Experts predicted that the 2019/20 flu season in the Northern Hemisphere would be bad. And then it wasn’t.
The COVID-19 pandemic hit, international travel stopped, physical distancing and the use of masks became more common, and the flu disappeared.
Rachel Baker, PhD, who studies public health and infectious diseases at Princeton University’s High Meadows Environmental Institute, said that in mid-March 2020, “there was a clear decline just as the closures were announced.” This was particularly evident in tropical places like Hawaii, which generally has persistent cases throughout the year rather than seasonal peaks alternating with periods when cases drop to zero.
Part of the sudden decline could 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 World Influenza Center at the Francis Institute. Crick in London, one of six influenza surveillance centers in the World Health Organization’s Global Influenza Surveillance and Response System.
More Americans chose to get the flu vaccine during the 2019/20 season than the previous year, according to the Centers for Disease Control and Prevention. The rate increased by 3.1 percentage points, and 48% of adults received the vaccine. However, vaccination rates for children remained the same.
Clearly, health measures designed to slow the transmission of COVID-19 can be just as effective in reducing the spread of influenza and other respiratory infections.
Baker and his colleagues estimate that COVID-19 precautions led to a 20% drop in respiratory syncytial virus (RSV) transmission in the United States last spring, and likely had a similar effect on influenza. And this might be an understatement because another Hong Kong study showed a 44% decrease in flu transmission after the introduction of public health measures.
“This is one of the clear signs that these interventions are working,” Baker said. “They are really efficient at stopping the spread of these viruses.”
“Surprisingly 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 also no “flu season to speak of” in the southern hemisphere, added Ian Barr, PhD, deputy director of the WHO Collaborating Center for Reference and Research on Influenza at the Peter Doherty Institute for Infections and Immunity in Melbourne, Australia.
This made things difficult for experts like Barr and McCauley, who contribute to the overall planning of which strains of influenza to include in seasonal flu vaccines. A decision becomes more complicated when there is a dearth of information about which strains are circulating in the population.
And it’s not like they’re not looking.
“We have tracked the planet to find where the viruses are,” Barr said. Overall, more samples were tested this year than last, but very little flu was found.
There were minor outbreaks in Bangladesh, India, Southeast Asia, West Africa, and Europe, but that was it. That left the WHO team with a dilemma as they developed the 2021/22 vaccine for the Northern Hemisphere: should they keep the vaccine strains the same, assuming those that were in circulation before the pandemic will return once it’s over? Or they will change. to reflect what they were seeing in small amounts around the world?
The team would have preferred to have more data to guide the difficult decision, McCauley explained. But in the end, they chose 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 only one, a strain of influenza A H3, for the one found in circulation.
What happens next is unclear.
Outbreaks next year
The models suggest that as population immunity declines over the course of the “lost season,” large outbreaks may become more likely after life returns to normal and public health measures implemented during the season are relieved. pandemic, Baker said. She predicts substantial outbreaks of RSV in future years, with a probable peak in the winter of 2021/22.
In the case of influenza, the picture is complicated by the transmissibility and evolutionary dynamics of circulating strains and the effect of vaccines, but a similar result is possible.
However, with fewer infections, influenza viruses have not had as many opportunities to mutate and evolve. That means that the strains that will appear in the future will likely be similar to those that were circulating before the pandemic, so the vaccines will remain effective.
And viruses will start from a low baseline, McCauley said, so they may not have time to spread to large numbers of people during the seasonal window.
It will likely take several years for flu numbers to return to normal levels, Barr said.
Still, “we have to plan for a worse season,” McCauley said. Even if the worst case scenario never happens, the effective vaccines that are available and widely used will help protect against a large flu outbreak. This is especially important at a time when the COVID-19 pandemic is not completely over because “the two together would be very difficult to handle,” he noted.
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