Flu scientists are also completely in the dark after two corona years in which the flu viruses didn’t stand a chance.

A few weeks ago, the European Center for Disease Prevention and Control (ECDC) sounded the alarm. Although the number of flu cases was still very limited in most European countries at that time, there turned out to be one country where flu viruses were already circulating much more frequently than people were used to for this time of year. It was about Croatia. And what ECDC saw there didn’t bode well for the rest of Europe. Because most flu infections in Croatia turned out to be caused by the infamous A(H3N2) type. The subtype is known to be especially dangerous for the elderly and is associated with lower vaccine effectiveness. “Early detection of the A(H3N2) suptype could be an indication that the upcoming flu season is going to be intense,” warned Pasi Penttinen, an epidemiologist at ECDC. “But,” he added immediately, “we can’t say for sure what the upcoming flu season will look like.”

The unknown

Even epidemiologists — trained to understand viruses and their journey — seem to be torn between hope and fear when it comes to the 2021/2022 flu season. “We don’t know what’s coming,” confirms flu researcher Robert de Vries, affiliated with Utrecht University.

Influenza-free flu seasons

Now it should be noted that flu seasons are always somewhat unpredictable. But where in previous years researchers could be somewhat guided by what they had seen in previous seasons, that is not the case this year. “It has actually been two years since we had a flu season,” says De Vries. It can of course be traced back to the corona measures that were widely embraced in the 2020/2021 flu season. Lockdowns, the mouth mask obligation, frantic hand washing and sneezing in the elbow: with all those measures we were not only bothering the coronavirus; the flu virus was also severely hampered by it. Even to such an extent that the number of flu infections in Europe during the (actually non-existing) flu season of 2020/2021 was no less than 99 percent lower than in previous years. “There were some outbreaks here and there, but there was no flu season.”

New questions

It is quite unparalleled and leads to new questions that remain unanswered to date and make it even more difficult to predict how the flu season will go. “For example, we have no idea how the population’s immunity is at the moment,” says De Vries. It also remains to be seen which flu variants – if they do manage to gain a foothold this winter – will emerge (see box).

Most flu cases are caused by two different flu viruses: influenza type A and influenza type B. These virus types can be divided into subtypes. For example, the A(H3N2) mentioned above is a subtype of the influenza type A virus. In the summer before the flu season, the World Health Organization tries to predict which subtypes will pose the greatest risk in winter and should be tackled with a flu shot. Scientists are looking at, among other things, which subtypes circulate most often in the southern hemisphere and spread most efficiently there. Dead versions of some of the most dangerous subtypes are then put into the flu shot to prepare people’s immune systems for an encounter with live virus particles belonging to these subtypes. But in years when the flu practically does not occur, it is not so easy to predict which variants we should fear this winter and which should be combated with the flu shot.

It should be clear: there is a lot of uncertainty. “We know almost nothing,” notes De Vries soberly. The fact that the ECDC – partly on the basis of observations in Croatia – nevertheless warns against a severe flu season can be explained at the same time. Because there are a number of factors that give cause for concern. “In Croatia you see that when the flu breaks out in a certain area, it can go very quickly.” The fact that it is precisely the A(H3N2) subtype that appears most often in Croatia does not help either. “Like the coronavirus, this subtype is a drift virus,” explains De Vries. “It mutates quite easily, which means that the flu shot (which also harbors the dead A(H3N2) subtype this year, ed.) protects a little less well against A(H3N2) than you would like.” Finally, the number of people who got the flu shot this year is disappointing. “Because the flu has practically not occurred for a few years, the flu shot has only been obtained very little and many may have lost their immunity.”

Get that shot!

Each and every one of these ingredients can lead to a severe flu season. And we cannot use that – certainly in combination with COVID-19. “If there is a major flu outbreak, the pressure on healthcare will increase even further. And that is the only reason why you should get the flu shot properly – especially if you are part of a risk group. That way you protect yourself, but also others.” In addition, we should also take a bit of an example from the Asians, according to De Vries. “In Asia, if you’re a little snotty, it’s very common to just stay at home. Or – if you really have to go out – wear a face mask. It wouldn’t be wrong if we started doing the same here.” Because you would almost forget it because of the flu-free flu seasons, but a flu infection is not nothing. Before the corona crisis, ECDC recorded between 15,000 and 70,000 flu-induced deaths in Europe each year. In the Netherlands, several hundred people usually die each year from the flu.

What this flu season will bring remains to be seen. And it can all work out in the end. Especially if we work from home en masse in the coming months, wear mouth caps and avoid large gatherings. Then a second flu-free flu season does not seem inconceivable. It’s a tempting scenario, but it doesn’t make the lives of flu researchers any easier. “We don’t know what’s coming next. And if anything is coming. But if nothing happens again this year, it will be even more difficult to predict what the flu season of 2022 will bring.” One thing is certain. “The flu will come back one day.”