Why isn’t the flu shot more effective?

According to Influenza vaccine - Wikipedia the influenza vaccine is generally about 40% or so effective each year and not higher than 60% in the last 16 years. Why is it that low, presumably even when we match strains well? Aren’t vaccines generally more effective?

The Wikipedia article has one sentence about antigenic drift, which seems to be due to mutations in the gene that encodes the surface proteins. So is it because flu is so common and therefore a larger populations size and more mutations accumulate within the population? So say lets take a more rare virus like polio. Because it is much more rare now, it doesn’t accumulate mutations on a relevant time scale and therefore vaccines are more effective? Is it that simple or there other reasons why flu vaccines are a 50/50 shot?


I’m not an expert but the aggregate effectiveness of the vaccine is not the same as the effectiveness of a particular vaccine against a particular virus. This matters because the “vaccine” aims at either 3 or 4 different influenza virus strains. I don’t know whether the 40-60% number accounts for that, but I think that in order for the vaccine to be ~100% effective, it would have to be a perfect match for all 4 strains, for that person. That sounds far-fetched to me.

Influenza viruses mutate pretty fast, hence antigenic drift and shift. SARS-CoV-2 apparently mutates 1/4 of the rate of the influenza viruses.

I’m not sure about that. I think it’s more likely a combination of the biology of replication of the viruses and the selective pressures that come into play.

1 Like

Vaccines are about treating the population, reducing the R0 to less than 1.

Seasonal flu has an R0 of about 1.28.

If was 40% effective, and everyone was vaccinated, that reduces the R0 by 40% to 0.76 Less than one, it will die out and we wouldn’t have a flu season. Most people would be susceptible, but not enough to allow it to spread.

However actual vaccination rates are about 45%, which reduces its effectiveness by 55%. So the R0 is about 1.05, greater than one, and influenza wont be contained each season.


For comparison, the COVID virus has an R0 of about 2.2, and the FDA has a target minimum efficacy of 50% for a vaccine. You should be able see very quickly the challenges ahead.

  1. That isn’t enough to control the virus, even if 100% of people got a vaccine. We really need something that works even better,

  2. Vaccination levels, including getting booster shots, is going to a major challenge. Almost everyone will need to be vaccinated for us to have a chance.

Hope that helps @Jordan.


I thought the question was about the reasons why a vaccine (or in this case, a flu shot, which is a combo vaccine aimed at 3-4 strains) has a particular effectiveness.


That is helpful.

Yes. Basically, somebody asked if the flu shot was ~ 40% effective, why would we think a COVID vaccine would be any better and why is it only ~40% effective in the first place if we can target the vaccine. I did some initial looking around Wikipedia but it wasn’t super clear so I thought I’d ask here.

At this point we don’t know yet. It’s a major risk point.

Though corona virus doesn’t mutate as quick as influenza, and it may not have as many zoonotic hosts. That’s helpful and hopeful, but doesn’t tell us for sure.

1 Like

I think the answer, in part, is that there’s just one target and it’s not (we hope) a rapidly moving target. There are a lot of big differences between the flu shot and a vaccine like the one we seek for SARS-CoV-2. The flu shot is based on predictions, in essence aiming at a target based on where we think it will be at the time. That alone makes the comparison unhelpful.

There are some interesting biological aspects to the flu vaccination effectiveness story, some of which are relevant to SARS-CoV-2; one issue seems to be immunosenescence, in which the immune systems of older people just don’t respond as well to vaccination. Also it seems that the flu shot is low-dose, and not adjuvanted. Higher doses might work better, and apparently adding adjuvants can help (adjuvants sensitize the immune system to encourage stronger responses). All of this suggests that effectiveness does mean, at least in part, aspects other than matching. Again, in the case of SARS-CoV-2, we hope that matching is not our biggest worry.


Since vaccines take a while to develop, they have to rely on predictions which aren’t always accurate. They are also designed on what the viral genome looks like at that moment. As you mention, influenza strains are a moving target so a vaccine designed for today’s virus may not be as efficacious 2 or 3 years from now.

All good points. Adjuvants may be helpful, but they can also result in adverse reactions which may discourage people from getting the flu shot. I’m sure this issue will raise its ugly head with the SARS-COV-2 vaccine as the conspiracy theory network gets fired up on FB and other sites.


Already has. Seems like natural immunity may not be long lasting, so a vaccine may be required to contain it. Herd immunity might never work.

Seems like every effective vaccine for COVID has non-life-threatening reactions, making people feel sick. It also seems like they all require to shots for long lasting immunity.

1 Like

Flu vaccines have also been shown to be short acting, so we may be looking at multiple shots in one year.

I still go to the dentist, even though I hate it. Hopefully, good sense will win the day.

I applaud your optimism. Unfortunately, my hopes are limited. The same people in my area who are claiming the virus is “hype” and who consider masks a violation of their “Constitutional rights” will in many cases refuse vaccinations. (Indeed, some are claiming that Fauci disses hydroxychloroquine because he will make millions of dollars on the vaccine. That is one of the reasons I fear that good sense will NOT win the day. Good sense is not in fashion where I live.)


My mother has gone down the conspiracy theory rabbit hole and she said some of those same things. Of course, no federal employee is allowed to profit off of their decisions as a public servant, so the accusations are baseless to begin with. Nonetheless, it is rather discouraging to see so many people throw logic and reason out the window.


Well, people generally view the pharma industry with mistrust.
In some cases, its deserved.
Overall I personally think finding a reliable/effective cure is more desirable than a vaccine. Atleast in India’s perspective…
Maybe for the US also.

Fauci is a public servant, not an employee of a big pharma company. Federal law forbids Fauci from profiting from any vaccine under his watch. He would go to jail for corruption.

A vaccine is a reliable and effective cure. Why do you think smallpox disappeared? Why do you think so few people get polio?

Why are you against vaccines?

Why isn’t the flu shot more effective? The answer, of course, is chickens. (And maybe ferrets)

OK, so the full answer also includes issues already discussed, like the variability of the virus and the reality that a multivalent vaccine may not be equally effective against all targets. But chickens are also part of the answer.

Most doses of the vaccine use viruses grown in chicken eggs. Not all strains grow equally well in these eggs, and serial passage in eggs can result in adaptations in the virus to replicate better in eggs. These adaptations can impact the parts of the virus that are antigenic, meaning that the egg-grown vaccine strains may have subtly different antigens than the virus circulating in the wild. This may mean that antibodies against the vaccine strains may not bind as well to the circulating viruses. The influenza A H3N2 strains are more likely to have antigenic changes when adapting to eggs, and so the vaccine tends to be less effective for H3N2.

As for ferrets, one facet of deciding which strains to include in a given year is looking at antigen cross-reactivity. Do antibodies elicited by infection to strain x bind to virus particles from strain y? If the answer is yes (based on a threshold on a quantitative assessment of binding) and y is circulating, then this supports the use of strain x in the vaccine. The antibodies for these tests often come from ferrets who have never been exposed to influenza viruses until they are deliberately infected with strain x for the test. But obviously ferrets are not humans; it is possible that differences in post-translational modification between ferrets and humans may mean that cross-reactivity could be different with human antibodies. Egg adaptation issues are known; I did not find direct evidence of actual cross-reactivity differences for humans and ferrets.


Am not against vaccines… one example of vaccines proving very effective in India is the Polio vaccine.
In the Indian scenario, it was useful because it gave immunity over a few doses for a person’s entire life. The government also took decades to eradicate the disease.

The challenge with a Covid vaccine will be the same as that for a flu vaccine… several doses might be required in a year and a large part of the population will have to be vaccinated in order to bring the infectivity below 1.0.
This is a big challenge in India because of the logistics challenge and the cost involved.
Hence my comment that an effective treatment of the disease might be the better solution in the Indian scenario.

Edit: Right now, a lot of governments are promoting the Covid vaccine as something that will help return the situation to normalcy. I just dont see that happening with the kind of vaccine that is likely to be provided (effective for shorter periods). As long as the case fatality rates are at above 2%, things will not return to normal.
Hence an effective treatment protocol is the key to restoring normalcy/returning to a pre-Covid lifestyle.

1 Like

We don’t have an effective treatment for COVID19. At best, some treatments lower mortality by about 10-20%. You are still going to have tons of hospitalizations, and you would need any treatment available all of the time all over the country to treat people who are already sick. That sounds like a really, really bad alternative. A treatment that prevents the infection to begin with is much easier on a healthcare system. Giving injections to healthy people that stay healthy is way less expensive than hospitals overrun with people on ventilators being treated with steroids.

Wait and see.

An effective vaccine will definitely help. There is no doubt about that.

I dont think a vaccine is going to lead to a normalization of the situation.
We will have to wait and see.

Yes, we will. I hope that people will withhold judgment until we do see the results of these studies.