Influenza vaccine protection in adults: it’s better than you think

It seems like every year we’re told that the timespan for which influenza (flu) vaccination provides us immunity is a little shorter. In the next few posts, I’ll have a look at some studies that have delved into this issue of influenza vaccine protection.


I wonder if we may be focusing on the wrong aspect of all this. Immunity will probably see many of us safely through an entire flu season unless the multiple circulating flu viruses mutate in such a way as to “escape” the immunity we’ve developed from vaccination. That happens. A universal flu vaccine will fix this failing up.

A study into persistence of flu immunity

A study from 2015 followed people vaccinated for two seasons in a row or given a placebo (saline). They were bled before and then at one, six, 12 and 18 months after receiving the flu vaccine.[1]

If the subjects acquired a confirmed flu infection during the study period they were excluded (this would not have been immunity due to vaccination). Any booster effect from a flu infection that was too mild to register as symptomatic enough, was at least partly accounted for by some pre-testing which created a kind of baseline.

Blood gets collected then sent to the lab to have cells removed for antibody testing
Photo by Hush Naidoo on Unsplash

Blood specimens were tested in the laboratory for antibodies developing to two flu virus surface proteins; hemagglutinin (HA) and neuraminidase (NA). The test is called hemagglutination-inhibition (HAI; see here for more of an overview on the HA version). In this study, it used viruses closely related to those in the vaccine which the subjects received. These were used to see how much antibody each subject produced after vaccination, at each of the time points.

AN ASIDE: The flu strains used were “closely related” as determined by lab testing. They reacted similarly to the vaccine strains when using ferret sera in HAI. What I’d like to see is the exact same strains used, together with human antisera. With other viruses that have a single-stranded genome (rather than multiple strands like an influenza virus), we generally don’t settle for “closely related” being good enough. This approach – used widely in flu’ville – overlooks contributions to human immunity made by viral components other than the surface-exposed HA and NA. This may or may not be important for understanding the strength and longevity of vaccine-induced protection.

Some shortcomings

The study didn’t look at the immune cell response to vaccination, just the antibodies produced. That’s okay, but it only tells part of the immunity story. The amount of antibody produced to flu strains is known to be related to the level of protection from wild flu strain infections.

The study also didn’t focus on those people at greatest risk of severe disease following influenza infection; young children, older adults, and those with certain pre-existing health conditions. They may respond differently to vaccination.

What the study did find

People receiving the placebo did not develop antibodies to the flu viruses contained in the vaccine. No surprise.

Vaccination offers solid protection against influenza
Photo by Samuel Zeller on Unsplash

Those who were vaccinated maintained (likely) protective levels of antibodies for 18 months – with a definite but only relativity small (2-fold) decline in levels over that time.

Subjects who had been vaccinated in the previous season saw a slightly smaller immune response to the more recent vaccination although the rate of antibody decline wasn’t really any different. This has been reported elsewhere and may be related to the very first symptomatic flu infection we had as children.[2,3,4] It’s a problem that we need to solve. Again, hopefully, future flu vaccines will overcome this issue.

A bit of a different story to what we’re used to?

I wonder if we may be overly confusing, or overly simplifying, the story of the Flu vaccine in order to drive uptake. The story of flu vaccination is certainly gaining more twists and turns as time and research uncover more aspects to consider.

Sure, it’s not the kind of vaccine that protects 95% of vaccinees from influenza every time. Absolutely, it’s not one shot that lasts a decade or more. But it is the only protective vaccine we have for the flu, and it does work better than nothing.

Study highlights

The study had a couple of big takeaways.

Most subjects respond to vaccination by developing antibodies.

I think we may talk too much about those who can’t respond or don’t respond. A caveat: because this study wasn’t focusing on the elderly it was unable to highlight that those over 65 years of age tend to have shorter immunity post-vaccination. [5] On the upside, there are now enhanced trivalent vaccines to cover the over 65s.[6,7]

Antibodies wane over time – but not as rapidly as some might suggest to you.

For adults, protection against influenza was likely to either completely prevent, or at least reduce the severity of, illness due to later flu virus infection for 18 months.

There is a lot of research around flu vaccination

Influenza is complex and the scientific literature reflects that. But perhaps the messaging isn’t keeping up.

Studies don’t always include all the variables when coming to conclusions around vaccine effectiveness but it is clear that current vaccines offer more benefit (less illness) than risk (most side effects = sore, warm arm and short term headache).

If you fall into a higher risk group though, make sure you talk to your Doctor about when to get your Flu shot and which one to get. In Australia, those groups include children from six months to five years of age, pregnant women, all Aboriginal and Torres Strait Islander people aged six months and older, people aged 65 years or older and anyone with underlying medical conditions that can increase the risk of influenza complications.[8]

My family is already vaccinated for the 2019 season. I will be soon as well.


  1. Persistence of Antibodies to Influenza Hemagglutinin and Neuraminidase Following One or Two Years of Influenza Vaccination
  2. Original Antigenic Sin: How First Exposure Shapes Lifelong Anti–Influenza Virus Immune Responses
  3. New flu strains and old antibodies: How sinful is ‘original antigenic sin’?
  4. From Original Antigenic Sin to the Universal Influenza Virus Vaccine
  5. Do antibody responses to the influenza vaccine persist year-round in the elderly? A systematic review and meta-analysis
  6. High-dose, immune-boosting or four-strain? A guide to flu vaccines for over-65s
  7. Here’s why the 2017 flu season was so bad

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