How Effective Is the Flu Vaccine?

The flu vaccine works.

How well does it work?

It depends…

How Effective Is the Flu Vaccine?

What does it depend on?

“The vaccine effectiveness of seasonal influenza vaccines is a measure of how well the seasonal influenza vaccine prevents influenza virus infection in the general population during a given influenza season.”

WHO on Vaccine effectiveness estimates for seasonal influenza vaccines

Is the flu virus that is going around the same strain that was picked to be in the flu vaccine?

Once upon a time, we didn't have flu vaccines to help keep us healthy.
Once upon a time, we didn’t have flu vaccines to help keep us healthy.

Has the flu virus drifted, even if it is the same strain that is in the flu vaccine, becoming different enough that your protective flu antibodies won’t recognize it?

Is the H3N2 strain of flu virus the predominate strain during the flu season? H3N2 predominant flu seasons are thought to be worse than others.

In general, the flu vaccine is going to be less effective in a season where there is a poor match between the circulating strain of flu virus that is getting people sick and the strain that is in the flu vaccine, especially if it is an H3N2 strain that has drifted.

That’s why, since the 2004-05 season, the average flu vaccine effectiveness has been about 41%.

How Effective Is This Year’s Flu Vaccine?

It’s probably also why, every year, we seem to hear the same questions:

  • Should I get a flu vaccine? – yes, definitely
  • Will we have enough flu vaccines? – while historically there have been some delays and shortages, we have a very good supply of flu vaccine this year, between 151 to 166 million doses
  • How effective is this year’s flu vaccine???

Unfortunately, we really won’t know the answer to that last question until this year’s flu season really gets going.

What about reports that the flu vaccine effectiveness will be as low as 10% this year?

It is important to note that those reports are not based on flu activity in the United States and it has been a long time since we have seen flu vaccine effectiveness that low – the 2004-05 flu season. That was the year that because of a drifted A(H3N2) virus, “only 5% of viruses from study participants were well matched to vaccine strains.”

The 10% number is instead based on reports of Australia’s flu season, in which early estimates found that the A(H3N2) component of the flu vaccine was only 10% effective. Importantly, the overall vaccine effectiveness was much higher. Including other strains, the flu vaccine in Australia was at least 33% effective this past year.

“In the temperate regions of the Southern Hemisphere, influenza activity typically occurs during April – September.”

CDC on Influenza Prevention: Information for Travelers

Couldn’t we see a drifted A(H3N2) virus this year?

Sure, especially since an A(H3N2) virus will likely be the dominant strain, but so far “data indicate that currently circulating viruses have not undergone significant antigenic drift.”

“It is difficult to predict which influenza viruses will predominate in the 2017–18 influenza season; however, in recent past seasons in which A(H3N2) viruses predominated, hospitalizations and deaths were more common, and the effectiveness of the vaccine was lower.”

CDC on Update: Influenza Activity — United States, October 1–November 25, 2017

Again, it is too early to predict how effective the flu vaccine will be, but based on an undrifted H3N2 virus that is matched to the vaccine, you might expect an effectiveness between 30 to 40%.

It might be less if theories about egg-adapted mutations are true and are a factor this year.

“…some currently circulating A(H3N2) viruses are less similar to egg-adapted viruses used for production of the majority of U.S. influenza vaccines.”

CDC on Update: Influenza Activity — United States, October 1–November 25, 2017

It is also important to keep in mind that vaccine effectiveness numbers from Australia and the United States don’t always match up.

For example, in 2009, Australia reported an interim flu vaccine effectiveness of just 9%, but in the United States, the flu vaccine ended up being 56% effective! On the other hand, in 2014, the flu vaccine worked fairly well in Australia, but vaccine effectiveness was found to be just 19% in the United States.

Vaccine Effectiveness by Year
Australia United States
2007 60% 2007-08 37%
2008 NE 2008-09 41%
2009 7% 2009-10 56%
2010 73% 2010-11 60%
2011 48% 2011-12 47%
2012 44% 2012-13 49%
2013 55% 2013-14 52%
2014  50% 2014-15 19%
2015  ?% 2015-16 48%
2016  ?% 2016-17 42%
2017 33% 2017-18 ?%

What does all of this mean?

Not much.

“This season’s flu vaccine includes the same H3N2 vaccine component as last season, and most circulating H3N2 viruses that have been tested in the United States this season are still similar to the H3N2 vaccine virus. Based on this data, CDC believes U.S. VE estimates from last season are likely to be a better predictor of the flu vaccine benefits to expect this season against circulating H3N2 viruses in the United States. This is assuming minimal change to circulating H3N2 viruses. However, because it is early in the season, CDC flu experts cannot predict which flu viruses will predominate. Estimates of the flu vaccine’s effectiveness against circulating flu viruses in the United States will be available later in the season.”

CDC on Frequently Asked Flu Questions 2017-2018 Influenza Season

The reports about what happened in Australia should not have made headlines beyond Australia.

As you should get your family vaccinated if you haven’t yet.

The Flu Vaccine Works

Getting a flu vaccine has many benefits with few risks and can:

  • reduce your chances of getting the flu
  • reduce the chances that your newborn gets the flu if you get your flu shot while pregnant
  • lead to milder symptoms if you do get the flu
  • reduce your risk of being hospitalized
  • reduce your risk of dying from the flu

And while it isn’t perfect, getting a flu vaccine is certainly better than remaining unprotected and simply taking your chances that you won’t get the flu and complications from the flu.

What to Know About Flu Vaccine Effectiveness

Although the effectiveness of the flu vaccine varies from year to year, depending on how well matched the vaccine is to circulating flu virus strains, which strains are dominant, and whether they have drifted, it is always a good idea to get vaccinated and protected.

More on Flu Vaccine Effectiveness

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