The flu shots that are being administered in the United States, as well as elsewhere in the northern hemisphere, are not reducing the risk of catching the dominant strain of the influenza virus (H3N2) that is circulating this year, the Centers for Disease Control and Prevention (CDC) in a March edition of its Morbidity and Mortality Weekly Report.

But top doctors told Health Policy Watch there is no cause for alarm.

Specifically, the efficacy of the formula at preventing mild or moderate cases of flu was estimated to be only around 16% this season, according to the interim CDC findings – meaning the risk of infection for a vaccinated person and unvaccinated person are almost equal – although vaccination can still protect much more against serious disease and death.

In the northern hemisphere, flu season usually hits its peak between December and February, but it can last until May, and in some years, new strains can start circulating at the end of the season as well.

“It is clearly disappointing in the sense that we would like to see it have a higher efficacy, but it is not surprising,” said Dr Edward Belongia, director of the Center for Clinical Epidemiology & Population Health at the Marshfield Clinic Research Institute, Wisconsin.  In Europe, the United States and “across the world” the dominant influenza strain this year has been the AH3N2 virus – comprising some 93% of all influenza viruses detected by the European Centers for Disease Control.

“We know flu vaccines in general do not work as well against H3N2 viruses [one of several influenza virus lineages circulating now] and there are variations from season to season. Some seasons we see quite a good match and, unfortunately, due to unpredictable variations in flu virus, sometimes we have a poor match.”

This year, in particular, scientists understood that a mismatch was likely, explained Dr. Kawsar R. Talaat, an associate professor in the International Health Department at Johns Hopkins University.

WHO’s Global Influenza Surveillance system

Twice a year, in September and February,  the World Health Organization and a global advisory group of experts examine circulating influenza strains picked up by the WHO Global Influenza Surveillance and Response System, which includes laboratories and research centers in 124 countries.  Based on that assessment, WHO issues recommendations for the composition of flu vaccines for the following winter seasons in the southern and northern hemispheres respectively.

But what this amounts to is basically a “guess what is going to be circulating the following winter based on what is circulating at the time,” Talaat said.

In February 2022, for instance, the WHO expert group already issued recommendations for the viral composition of influenza vaccines for the next 2022-23 winter flu season in the northern hemisphere – based on what has been observed circulating over the past six months.

In 2020-2021, however, the flu season in both northern and southern hemispheres was historically mild as a result of COVID restrictions, Talaat said.  As a result, the February 2021 WHO recommendations for this year’s season in the global north were even more flawed than usual.

“It is an inaccurate science already,” Talaat told Health Policy Watch. “To do it without an existing flu makes it even harder.”

However, five years ago, even in the absence of COVID, there was a similarly mismatched season, Talaat pointed out.

“We should not be alarmed by this and there is no reason to panic,” she said.  And even with the lower efficacy of the flu shot, those at highest risk should still get the jab, she stressed.

The CDC said so too, recommending the shot for anyone over six months so long as the virus is circulating. The agency report stressed that the vaccine could still prevent serious disease, hospitalization and death.

“The same groups that are at risk for having severe COVID infection should get vaccinated because it could protect these individuals and keep hospitals from having to deal with both severe COVID and flu patients at the same time,” Belongia stressed.

Is there anything that can be done?

The long-term solution, he said, is to develop a universal flu vaccine that protects against all strains. A universal flu vaccine, according to the National Institute of Allergy and Infectious Diseases (NIAID) would provide “robust, long-lasting protection against multiple subtypes of flu, rather than a select few. Current WHO-recommended flu vaccines typically protect against three or four major flu strains. Such a vaccine would eliminate the need to update and administer the seasonal flu vaccine each year and could provide protection against newly emerging flu strains, potentially including those that could cause a flu pandemic.”

It would be at least 75% effective for all age groups and protect against group I and II influenza A viruses.

“That is somewhere down the road,” Belongia said. “It won’t happen in the next year or two, but it is a very active area of research.”

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