Let’s talk flu vaccine efficacy.

Anti-vaxxers like to throw around low numbers. Pro-vaxxers put out the high numbers. Public Health experts try to tell the whole story.

First, let’s talk about the flu virus. Influenza virus mutates. A lot. There are many strains, and in any given year 2–4 strains could be circulating. Scientists predict which strains will circulate in the next flu season based on what has circulated in other parts of the world. They develop a vaccine that targets the specific antigens on those strains. It’s not a guess, it’s based on science and field data. Sometimes, they get it correct. More often, they get it partially correct. And some years, like last year, the four flu strains chosen were correct but one strain tricked us. It mutated again before it was circulating in the northern hemisphere. As viruses go, influenza is a “smart” one — it keeps mutating to avoid detection.

Every flu vaccine will have multiple strains. Most years, the vaccine does really well targeting 2–3 of those strains. Some years, one strain breaks out. The 2018–2019 flu vaccine contained four* strains:

· H1N1, an A/Michigan/45/2015-like virus

· H3N2, an A/Singapore/INFIMH-16–0019/2016-like virus

· Victoria, a B/Colorado/06/2017-like virus

· Yamagata, a B/Phuket/3073/2013-like virus

Influenza A viruses accounted for 98% of the flu circulating last year. Of these, it was about half split between H1N1 and H3N2. Influenza B strains accounted for only 2%.

Overall efficacy, for all four strains, was about 29–30%. This is much lower than we’d like, of course, but that doesn’t mean the vaccine didn’t work. Let’s break it down by age. Those most at risk for serious complications and death, kids aged 6 months to 8 years were protected at a rate of 49% against ALL four strains. Perfect? No. But if it’s a choice between zero and 49% protection, I’ll take the latter. Unfortunately, kids aged 9–17 years had a vaccine efficacy (VE) as low as 6% — i.e., virtually none. For young adults (18–49), VE was 25%. For those over 50, it dropped to 12%.

Does that mean it was useless? No. The course of illness was generally greatly reduced for those who were vaccinated.

And that was overall. But remember the vaccine had four strains. For A/H1N1, VE was 44%. For the B strains, we saw so little of them, we don’t include the data.

So why was the overall efficacy so low? A/H3N2 was the culprit. Scientists predicted the correct strain. It was in the vaccine. A/H3N2 just mutated faster than it normally would have, too fast for our vaccine to catch up with it. Did the vaccine work? Only a little against this strain (9%). Anti-vaxxers like to focus on that number. But it doesn’t tell the whole story. Remember A/H1N1 was 44% effective.

Do we just stop vaccinating against flu because one of the four strains failed one year? No. It means we continue the work to identify the new strains circulating, adapting the vaccine as needed — as possible. More importantly, if you have 75% of the population vaccinated, even a lower efficacy helps to prevent the virus from spreading. If you have zero vaccinated, the virus WILL spread, hard and fast and furious.

Vaccines don’t have to work with 100% efficacy to save lives. Was it hard to deal with an hysterical child afraid of needles? Yup. Did I wish I could just say no? Sure. But it’s a lot harder to see a child spend a week in PICU on a ventilator because I didn’t give him a vaccine.


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*The trivalent vaccine omitted the second B strain (Yamaguta)

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