By skyforbes TL;DR – this year is giving influenza H3N2, somewhat escaped our vaccine because of quick mutation, but still get the vaccine now, it's not too late!
Preface: I am a doctor working in the emergency departments and recently saw a huge flood of patients with influenza. I was very curious about why influenza was blowing up this season, so I did some research. I'll summarize the main points and leave references below:
1) Influenza H3N2
This year's influenza wave is largely driven by the Influenza A (H3N2) strain. There are 3 main strains: influenza A (H1N1), influenza A (H3N2), and influenza B (much less common). Previous years were H1N1 dominant and so perhaps by virtue, there may have been some residual protection or not (hard to say), but this year's wave is driven by influenza A (H3N2). As of August 24, 2025, 71% of influenza A detected infections were H3N2, making it a large proportion of the current wave. As well, as of early December, percent positivity of influenza A is 26% i.e. if you have viral symptoms, very likely that it's influenza A, much bigger than the past 3 seasons. For context, COVID-19 is 4.6% right now.
H3N2 is typically more severe in illness, especially for the young and elderly, and is associated with higher rates of hospitalization and deaths. We also probably have not developed as much resistance to it too since prior years were H1N1-dominant. We are tragically seeing many deaths of children and elderly people in the hospitals this season. I am expecting a lot more people in hospital over the new years especially after everyone starts getting together for Christmas.
2) Vaccine uptake and efficacy
This year, the H3N2 virus unexpectedly had a major genetic shift in August 2025, creating a new subclade called the A(H3N2) K subclade viruses (previous ones were the J subclade). Vaccine development typically uses data collected by February of the year for the northern hemisphere, so the current vaccine protects against what the expected dominant influenza strains are for the year for H1N1, H3N2, and B. You can imagine that if H3N2 suddenly gets a jump on the world by mutating after vaccine development has started, then it can evade some of that vaccine protective effect.
Further, vaccine uptake has been dismal year after year, with 2023-2024 being 43%, similar to prior years, and likely the same this year. This is multifactorial: the wave of influenza hit us 3-9 weeks earlier compared to the past 2 years so many people haven't gotten their vaccines yet; and lots of anti-vaxx sentiment nationally as well as imported from south of the border. In addition, the vaccine development largely depends on data collected globally, and our biggest neighbour, the US with the CDC, was a major source of information for virus data collection and surveillance. However, with RFK Jr. in office being anti-vaxx and cutting ~25% of the CDC work force, we've now lost that information and as such, this impacts not only the US, but also our own health and health care system.
3) What should I do? Should I still get the vaccine even though it's less effective/if I've already gotten the flu?
Great question, average Ontario redditor!
Yes, generally, I would still highly recommend the seasonal influenza vaccine.
– Current vaccine effectiveness estimates show the current vaccine is 70 to 75% effective at preventing hospital attendance in children aged 2 to 17 years and 30 to 40% effective in adults.
– Even if you already had "the flu": a) your "flu" could have been any circulating respiratory virus like rhinovirus, enterovirus, RSV, human metapneumovirus, seasonal coronavirus, COVID-19, etc., and getting sick from those do not give you natural immunity to influenza; b) Even if you were actually infected with influenza, there are 3 strains and getting infected with one of them still puts you at risk of getting the other 2, which would still be a wild ride to go through. The vaccine is still safe and effective even after you've caught influenza, just get the shot a few weeks later once you're feeling better.
– For an immunocompetent person, it typically takes 7-14 days for your vaccine to reach peak effect, so don't expect immediate protection right away. Still get it early and early for future years.
– The vaccine is safe to take. You will not get influenza from the influenza vaccine; it is an inactivated virus, do not propagate this claim god damn it. Egg allergy is also no longer a contraindication to the vaccine.
– What about people preferring to get natural immunity? See above points about how it won't protect you as much from the other ones, plus I have seen on personal accounts several negative outcomes from actually catching influenza: higher risk of myocarditis, subsequent bacterial pneumonia (especially with MSSA/MRSA), acute respiratory distress syndrome, acute kidney injuries, etc.
– Plus, having influenza and viral infections in general puts you at risk of developing dementia and cardiovascular disease later down the line. Your body fights viral infections and probably puts a bunch of tau proteins into your brain, leading to dementia and neurodegenerative disease. Even if you have mild cases of the flu or other viruses, that stuff builds up in your brain.
References below in comments.
EDIT:
I'm seeing a lot of common themes in the comments and wanted to pin them.
1. A lot of people are getting sick and quite symptomatic. To all of you, I am hoping for you and your families' speedy recovery.
2. The vaccine has decreased efficacy, is it worth it still to get??
– The 30-40% number I quoted is the reduction in HOSPITALIZATION RISK. That means that it will protect you from getting totally rocked by the virus to the point of having to get hospitalized. However, symptomatically, it will likely protect you to a higher percentage degree.
– Compared to the previous 3 years, this year is 30-40% reduction in hospitalization for adults, but the previous 3 years' efficacies were 40-50%, which is still kind of close to what it is typically. You're never going to get 100%, but that's not biologically possible as people will not be able to mount perfect responses, nor is it the point of the vaccine. What it can do is prevent you from getting sick to the point of becoming hospitalized, and you will still likely get symptomatic benefit (i.e. feel less bad, recover a bit quicker, fewer complications).
3. What if I'm pregnant? Breastfeeding?
– The influenza vaccine is safe in pregnant women and is highly recommended. Fevers in the case of underlying infection/autoimmune disease would likely impact the fetus (both from the fever and from the underlying illness itself), but typically the vaccine fever is self-limited, unlikely to cause harm to the baby, and also protects you from actually getting influenza, which in itself increases the risk of adverse fetal outcomes. Influenza during pregnancy increases the risk of adverse fetal outcomes, including congenital anomalies (such as cleft lip, neural tube defects, hydrocephaly, and congenital heart defects), preterm birth, low birth weight, small-for-gestational-age infants, pregnancy loss, and fetal death. It also increases the risk of MATERNAL death too.
– Basically, talk to your doctor, but if you don't have any other major issues or contraindications, I would generally urge people to get it (inactivated influenza vaccine, NOT the live attenuated vaccine, which is contraindicated in pregnancy).
– The vaccine is also safe for breastfeeding at any point in time.
4. When can I give it to my children? What if they're <1 year old?
– All kids 6 months old and older should get the flu vaccine every year, unless there is a medical reason not to. Babies under 6 months old are too young to get the flu shot, but they'll get some protection if their parent got the flu shot while they were pregnant. Young children have a high burden of influenza illness with very high risk of serious infection and hospitalization among the youngest. Because young children are less likely to have had prior exposure to an influenza virus, a 2-dose schedule is required to achieve protection for those less than 9 years of age that are previously unvaccinated.
5. If the current flu shot isn't protecting against influenza A, why get the flu shot? By how much does it reduce symptoms of influenza A?
Great question and I love that I can answer these questions in this forum.
– Vaccine protection is not a 0% or 100%, black or white, heads or tails. It is a spectrum of protection. In fact, we actually see some early data suggesting that the vaccine is still working quite well (see above, preventing getting yourself so sick to the point of being hospitalized by 30-40% even with the mismatch, compared to the usual 40-50% we typically expect), and it is even more effective in children (seeing ~70% vaccine efficacy in preventing hospitalization). There is also a large body of evidence showing that it will reduce the major complications of severe infection as well, such as heart attacks, strokes, severe pneumonia, getting sick to the point of needing ICU admission, and death.
– The percent of symptom reduction is different for everyone. If you have more chronic illnesses, you will likely still get quite sick, and if you're a young healthy person, it may very well still reduce your symptom burden and duration of illness. I wouldn't be able to give you a percent on this.