The Economist explains
Will the covid-19 pandemic make this year’s flu season worse?
Low influenza infections last year mean immunity is low and producing a vaccine tricky
A YEAR AGO, the northern hemisphere was entering its first winter in the shadow of the covid-19 pandemic. Public-health officials asked people to wear face masks, employ social distancing, wash their hands regularly, stay at home, keep children out of schools and reduce their travel on public transport. These actions to slow the spread of covid-19 had a subsidiary effect, reducing the spread of other respiratory viruses such as influenza. The World Health Organisation reported that less than 0.2% of the respiratory swab specimens tested from people around the world between September 2020 and January 2021 were positive for influenza viruses (see chart). In contrast, the average during the same reporting period of the three previous seasons (2017-2020) was 17%.
Far from being beneficial, however, the lack of an influenza season in 2020-21 could have stored up an even bigger problem for 2021-22: the flu could strike back in the coming months with a vengeance. Outbreaks could occur sooner, last longer and affect many more people than usual. How likely is that, and how might it be avoided?
The annual seasonal flu is caused by a group of viruses that circulate around the world in birds, humans and other mammals. In the northern hemisphere the season usually starts around October and peaks the following January or February. There are two virus types of which health experts keep track. Influenza A viruses originate in birds and pigs and their H1N1 and H3N2 subtypes (named according to the specific structures of two proteins on their surfaces, haemagglutinin and neuraminidase) can infect humans. Influenza B is a human virus. As they mutate, these bugs are exceptionally good at reconfiguring the proteins that sit on their surfaces, making them more capable of slipping past a person’s immune defences, even if they have been infected with flu before. That means vaccines against influenza need to be tweaked and re-administered every year.
It takes at least six months for drug companies to create, test and manufacture large quantities of flu vaccines, so they need to decide well ahead of the flu season which versions of which viruses need to be included in the vaccines for an upcoming winter. They do this using data from the Global Influenza Surveillance and Response System, a network of laboratories and public-health institutions in 123 countries that collect respiratory samples throughout the year.
There are two reasons why the coming flu season might be worse than normal. First, because there have been so few influenza virus samples collected over the past year, it is possible that some threatening types have gone undetected. According to Britain’s Academy of Medical Sciences, when this happens “[vaccine] effectiveness against both infection and severe disease can be markedly reduced, resulting in more severe epidemics.” Second, because so few people have been exposed to the circulating flu viruses in the past year, the levels of natural immunity in populations will be relatively low.
How can the northern hemisphere avoid a “twindemic” of a simultaneous surge in covid-19 and influenza? One important step will be to get as many vaccines (for both diseases) into as many arms as possible. Though protection is never total, vaccination does a useful job of reducing the number of susceptible people in a population. Since natural immunity to flu around the world is probably at its lowest for years, immunity via vaccines will need to make up the shortfall. Immunising infants and schoolchildren against flu will also be important, given the role these groups play in spreading the virus to older people.
It would also help if doctors had routine access to tests for influenza. Identifying the cause of an infection is useful—timely use of antivirals such as oseltamivir, for example, can reduce the number of days a person is sick with the disease and also keep people out of hospitals.
The final task will fall to the public. Though government restrictions to reduce physical contacts between people may no longer be in force in many parts of the world, people can still practise respiratory hygiene by wearing masks in crowded indoor environments, working from home where possible and socialising outdoors.
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Why do new SARS-CoV-2 variants spread more easily?
Why official covid-19 deaths do not capture the pandemic’s true toll
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