The epidemic of flu It arrived this year in Spain and Europe unusually early. In Spain, the peak already exceeds that of the last three seasons and the incidence, which during the first week of December – the last analyzed – increased from 78.3 to 164.6 cases per 100,000 inhabitants, surpasses last year’s highs seven weeks early.
The cause of this advance is the new variant, the subclade K of influenza A (H3N2), which has several specific genetic changes in the protein that coats the virus, called hemagglutinin, making it a strain with a partial ability to evade immunity conferred by the vaccine or by previous years’ infections of different variants. This results in a increased transmission but, as yet, it has not been proven to cause more severe cases or hospitalizationsas explained by the spokesperson for infectious diseases of the Spanish Society of Family and Community Medicine (semFYC), José María Molero.
Changes in the outer shell of the K subclade influence the effectiveness of the flu vaccine we received and therefore the vaccinated person is more likely to be less protected. Reduces the effectiveness of the vaccine. These changes have been shown to result in faster transmission of the virus from one patient to another, likely due to immune evasion. And this also contributes to the fact that it spreads unusually quickly and, therefore, the peak of infection is ahead of what we usually find in flu epidemics or what we found before Covid”, explains Molero, who emphasizes at the same time that “it has not been shown that it is a more serious virus or that it produces more cases of complications or hospitalizations”.
Severity of the K variant
In the United Kingdom, health authorities are facing what they have described as “the worst possible scenario” in this month of December. According to the English press, some schools have closed or partially reduced their activity and some residences have advised against visits due to the increase in flu cases. Subclade K is to blame: “The majority of viruses sequenced to date belong to the genetic clade J.2.4.1 (also known as subclade K),” says the latest flu surveillance report from the UK Health Security Agency.
Molero explains that we must qualify when we talk about aggressiveness because “it can be interpreted in two ways”. One thing is that it is described as more aggressive because it spreads more quickly, which is “effectively what stands out from this subclade”, which “is mainly characterized by a great capacity to spread and therefore impact on infections and the health system”. And another problem is that this gives rise to serious cases, which have not yet been proven.
Risks of infection
However, Molero declares that “The severity of an infection depends not only on the characteristics of the virus, but also on the population it affects and the number of people affected.“When a virus spreads more easily,” the doctor continues, “it is likely that, without the virus itself being more serious, the cases of patients with complications will increase, but not because the virus is more serious, but because the possibility of being infected and transmitting it actually increases.
“It was found that the contagion potential of the K subclade is somewhat higher than that of other H3N2 clades that do not have these mutations. It increases the chances of infecting or transmitting the infection by approximately 20-25% compared to other subtypes. of the H3N2 influenza virus family. This possibility of transmitting more easily is what caused the peak of infections to be several weeks ahead of what we had planned or what we expected for other years,” explains Molero.
Should we confine?
In Spain, This Thursday the Community of Madrid recommended the use of masks in retirement homes, hospitals and health centers and also advised not to coincide with more than one visitor per resident at the same time, with the aim of avoiding the spread of influenza among the elderly due to the high circulation of the virus. In addition, in Catalonia, they are compulsory in hospitals and residences from this Wednesday. Outside of Spain, in the United Kingdom, some schools have sent their students home as a “fire break” or partially reduced their in-person activity to stop infections.
Given the reestablishment of certain preventive measures against the Covid-19 pandemic, it is worth asking whether the current explosion of influenza would require a measure such as confinement. Dr. Molero rules it out: “Decree a total lockdown or significant social restrictions at the national level due to the increase in flu cases This does not seem to be an effective measure“.
“With this public health measure, you have to think about what you gain and what you lose. So far, what we are seeing is that cases have accelerated numerically, the estimated peak has been brought forward by more than a month, but we do not see a particularly serious subclade, which would produce a mortality problem,” the semFYC spokesperson said.
The latest report from the Carlos III Health Institute, published this Thursday, estimates the rate of admissions due to influenza in 7.5 cases per 100,000 inhabitants (compared to four cases the previous week). In terms of severity, since the start of this season, cases hospitalized for influenza have occurred “19.3% pneumonia, 4.4% intensive care admissions and 4.6% deaths” says the document.
For Molero, at present, “it does not appear that there is a clinical reason which currently justifies the need” to decree a confinement. A measure which, he adds, is also taken when “it is impossible to act at the clinical level against the infection”. The flu, in this case, is a known disease with treatment, unlike what happened when the covid pandemic arrived. “I think that neither at this time nor during the flu epidemic is this a cost-effective measure.”
The peak, for Christmas. And then how will it go down?
Determining when the peak of infections will be reached “is complex because it will depend a lot on the speed of spread of this K subclade of H3N2”. However, Molero, as he already mentioned 20 minutes at the end of November, affirms that “the most probable”, given that no worsening of the disease has yet been observed, is that the peak is estimated for “the last week of the year, a little before or a little after”. That is, in the middle of Christmas celebrations, when there are many family gatherings and people do not see each other regularly, which can lead to increased transmission.
In this sense, Molero asks “to take into account the impact that all the social celebrations of this time will have, in addition to the cold, which also accelerates transmission.” All these circumstances may “push the peak a little before the end of the year, but the most likely scenario is that it will be the last week of 2025.”
Then, continues the doctor, “we will see how the peak goes down, because influenza B or A(H1N1) can then maintain its circulation, which is now virtually replaced by subclade K, and perhaps instead of a faster decline we have a slower decline, even with small plateaus and small increments by other influenza viruses which then normalize. “We’re going to see him.”
What precautions should you take?
The semFYC spokesperson recalls the measures that should be taken by anyone with flu symptoms or a respiratory infection (cough, nasal congestion, fever, general discomfort, etc.). An infected patient “must be careful not to contaminate others, whether they are vulnerable or not”. As? “Maintain a rest at home appropriate in the maximum phase, the first three or four days, trying to avoid social contacts and, if you must have social contacts, logically, with face mask“. The specialist adds to these measures frequent hand washing, maintaining a distance of one and a half meters to avoid contagion to others and teleworking when possible.
Molero also emphasizes the flu vaccine, which “is still effective, less than would be expected for subclade K, but it protects against other H3N2 variants, against the H1N1 A virus which is also circulating, and against influenza B.”Vaccination remains effective because after two weeks, it already has a fairly high level of protection and it is predictable that we still have about two months of flu cases left.
Vaccine effectiveness
Regarding the effectiveness of the vaccine, Molero maintains that the changes presented by the K subclade can reduce it, but that it remains effective against the production of serious diseases or the development of complications. “It was found that, although the vaccine introduced this year does not correspond 100% to the K subclade, a component of H3N2 was introduced in 2023, which is the one that is maintained in 2025, which already provides some protection against severe disease in cases where H3N2 evolves into something similar to what we have now. That is, We were already vaccinated with a strain of H3N2 that, while not exactly faithful to the K subclade, was found to have mechanisms to generate immunity.antibodies and cells, which protect against serious illness, which normally causes pneumonia and requires admission and, sometimes, the intensive care unit.
When to go to the doctor?
Molero warns that the flu is already straining health services. At the moment, according to semFYC, Madrid, Aragon and Catalonia are the communities with the most cases, while Andalusia, Murcia or Navarre have “a little less”. Either way, he notes, “in the end, unfortunately, all communities will experience this infection and then equilibrate. Some communities will reach the maximum peak earlier and others will reach it later.”
To avoid saturation of emergencies, the doctor indicates that the characteristics of the flu should alarm the population. In this way, you must consult a doctor if after three or four days the fever persists above 38 degrees or 38.5º. Also if you notice “sudden worsening” or no improvement after three or four days. Pain on a specific side that prevents breathing, pain when breathing, or an intense condition that prevents hydration or adequate food intake are also reasons to see a doctor.
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