You might think that many people are sneezing, coughing, and sick with COVID-19 and the flu at the moment, but it’s nothing compared to the extreme flu season of 2017.
It lasted well into 2018, knocked families down, and had twice the mortality rate and led to twice as many hospitalized Danes, especially the elderly, than the normal seasonal flu.
It was one of the toughest and longest flu seasons in years, and what was particularly unique about this epidemic was that it was dominated by one of the four main strains of the influenza virus; the so-called B/Yamagata.
Now you might wonder, why on earth should I know this when I subscribe to Politiken and not the Journal of the Danish Medical Association?
The answer is as simple as it is fascinating.
At least, if you ask senior researcher Ramona Trebbien, head of the Influenza and Respiratory Virus Section at the Statens Serum Institut.
»Influenza B/Yamagata has simply disappeared. It’s completely gone«, she says.
In Denmark, the last two people were diagnosed with the special flu variant in March 2020.
»But it’s not a Danish phenomenon. It’s global. It’s gone«, she says.
»Nowhere, where influenza virus and flu season are nationally monitored – whether it’s in the northern hemisphere, where the infection spreads in our winter months, or in the southern hemisphere, where they get the flu when we have summer – has anyone detected influenza B/Yamagata since the spring of 2020«.
Virologists, however, are hesitant to use the word eradicated, as it is possible that the virus – which constantly needs to find a new host to infect in order to survive – has gone under the radar of national systems for several years.
»There are, after all, countries that do not monitor as we do in the western and developed parts of the world, but I don’t believe that explanation myself. The virus has to travel from the northern to the southern hemisphere every year and then return to us. It would be very strange if B/Yamagata hadn’t been found in the systems somewhere in the world since the spring of 2020. If it is still spreading«, she says.
The virus still exists in secure laboratories for research purposes. In the event of an accident, the virus could escape and spread again.
»That risk is infinitely minimal«, she says.
Certain flu vaccines consist of live attenuated viruses, and until recently, there were also vaccines with the special, disappeared B strain. In some countries, a few cases of B/Yamagata were found – after vaccination. Not the real virus, but a cultivated, weakened variant that can neither infect nor cause disease.
»Therefore, WHO does not consider it a finding. Those few cases don’t count«, says Ramona Trebbien, who is the person reporting Denmark’s flu findings to the health organization.
»So, in layman’s terms, we can say that B/Yamagata is eradicated. We haven’t seen it for over five years«, she says.
Different viruses
It is typically two main influenza A viruses that cause seasonal flu – A/H1N1 and A/H3N2.
Influenza A viruses also exist in animal reservoirs, primarily in birds and pigs, where they can sometimes jump from humans to animals and vice versa. They evolve quickly and change from year to year, driving the spread of seasonal flu.
The two influenza B viruses, on the other hand, are genetically different when Ramona Trebbien and colleagues closely examine them in laboratories. They evolve more slowly, and outbreaks of B virus infection are seen to a lesser extent and usually at the end of the flu season.
»The B variant Victoria typically affects children and young people. It doesn’t change much, so we often have antibodies against it when we are exposed to it again later in life. Therefore, it doesn’t affect the elderly as much or as severely because they have some immunity«.
»B/Yamagata – the one that is now gone – often affected the elderly because they didn’t have immunity from previous infections to the same extent. Also in 2017-2018, when it was really hard on the elderly«.
The last major outbreak with the special B strain ensured a pronounced global immunity, and in the years following, it spread only at a very low level.
And then something happened.
The cause
A completely new virus, which no human had been infected with – a coronavirus – spread rapidly from China, pushing the flu into the background.
»The global lockdowns in the first months of 2020 – in March in Denmark – reduced all influenza activity to a minimum. Our theory is that B/Yamagata couldn’t spread due to the lockdowns and the already high immunity in the population from the outbreak a few years earlier«.
»And when society reopened, we could no longer trace it«, says Ramona Trebbien.
It may be of interest to researchers, but does it matter to the population?
»Yes, I think it does. It matters for the burden of infection that it’s gone. Especially for the potential pressure on hospitals, as B/Yamagata previously caused more illness among the elderly, who are more often hospitalized due to influenza. And now it’s gone«, she says.
There are different types of vaccines against influenza. In 2023, WHO stopped recommending the so-called quadrivalent vaccine, which also protected against B/Yamagata.
However, testing for the virus type must continue.
For it might come back.
Ramona Trebbien, however, doesn’t really believe that.
»Now five years have passed, and the longer time goes by, the less likely it is that it will reappear«, she says.
