The Flu Season From Hell: Why It's So Bad This Year
The United States is currently experiencing its worst flu season in a quarter of a century, according to the Centers for Disease Control and Prevention (CDC). As of January 3rd, the CDC estimates a staggering 15 million illnesses, 180,000 hospitalizations, and 7,400 deaths. But here's where it gets controversial: a new virus strain called subclade K might be to blame.
Yonatan Grad, a professor of immunology and infectious diseases at Harvard's School of Public Health, believes this new strain has "antigenic differences" from the strains used in this year's flu vaccine. In a recent interview, Grad sheds light on why some flu seasons are more severe than others and what we can do to protect ourselves.
Why Are Some Flu Seasons Worse?
The severity of a flu season depends on two key factors. Firstly, the susceptibility of the population to the circulating influenza strain. Immunity is gained through past infections and vaccinations, but this protection can fade over time, especially as the virus evolves. Secondly, human behavior plays a crucial role. During the COVID-19 lockdowns, with limited interaction and virus transmission, influenza cases were minimal.
Understanding the Strain Circulating This Year
Influenza can be classified into two main classes: A and B. Historically, two strains of influenza B circulated annually, one from the Victoria lineage and another from the Yamagata lineage. Interestingly, Yamagata seems to have become extinct during the pandemic.
Within influenza A, H3N2 and H1N1 have been the dominant subtypes for nearly 50 years. Over the last century, several subtypes of influenza A have circulated among humans. The 1918 flu pandemic was caused by the H1N1 subtype, which seasonally circulated until 1957 when an H2N2 pandemic replaced it. In 1968, an H3N2 pandemic emerged, and this subtype has been the seasonal flu ever since, with H2N2 not seen since.
Typically, one influenza A subtype dominates a season, with influenza B also circulating. This year, H3N2 is the dominant strain. While flu B tends to be milder, it can still cause severe illness. H3N2 is believed to be more severe, but the reasons for this are not fully understood.
Grad delves into the history of influenza A, suggesting that the first flu exposure may influence an individual's immune response throughout their life. This could explain why H3N2 appears more severe for those born before 1968, when H3N2 started circulating.
Why Didn't This Year's Flu Vaccine Match?
The H3N2 strain used in the vaccine, a subclade J.2 virus, appears antigenically different from the circulating H3N2 strain, which belongs to subclade K. This raises concerns about the vaccine's effectiveness against the circulating strain.
However, a recent study shows that the vaccine is not as ineffective as initially thought. It appears that people do develop an immune response to subclade K after vaccination. Additionally, early estimates of flu vaccine effectiveness this season are similar to what we typically see for H3N2 strains.
How Are Flu Strains Selected for the Vaccine?
The decision to include specific strains in the vaccine is made months before the flu season, usually in February for the northern hemisphere. This early decision-making is necessary because most seasonal influenza vaccines are manufactured using chicken eggs, a process that takes a long time. Subclade K emerged in the spring of 2025, after the strains for the season's vaccines were already set.
This situation raises questions about the potential benefits of faster influenza vaccine technologies, which could allow for later strain selection and potentially avoid such mismatches. mRNA vaccines for influenza might have been a solution, but federal support for mRNA vaccine research has been cut, so we may not see progress in this area anytime soon.
Is It Too Late for a Flu Shot?
No, it's not too late to get a flu shot, especially as the flu is expected to persist for the next few months. It takes about two weeks after vaccination for protection to take effect, and this protection gradually decreases over the following four to six months. Typically, we recommend getting vaccinated around Halloween to maximize protection during the peak of the flu season, which is usually mid-winter.
In addition to vaccination, Grad recommends general precautions to protect against respiratory viruses, such as masking, hand hygiene, and avoiding crowded, poorly ventilated spaces. These measures, familiar to many from the COVID-19 pandemic, can further reduce the risk of flu transmission.
COVID-19 Trends This Winter
The relatively low surge of COVID-19 cases this winter is likely due to the population's immunity against the circulating SARS-CoV-2 strains, resulting from past infections and vaccinations. As population immunity wanes and new variants emerge, we may face another wave of COVID-19, leading to periodic surges in the future.
