The flu vaccine works.
How well does it work?
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?
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 usually don’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.
|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?
“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.
Folks should have waited for the yearly report on flu vaccine effectiveness from the CDC, which usually comes out in the middle of February. This year, that report states that:
- the overall adjusted vaccine effectiveness against influenza A and influenza B virus infection associated with medically attended acute respiratory illness was 36%
- vaccine effectiveness was estimated to be 25% against illness caused by influenza A(H3N2) virus, 67% against A(H1N1)pdm09 viruses, and 42% against influenza B viruses
- the flu vaccine offered statistically significant protection against medically attended influenza among adults aged 18–49 years with an adjusted vaccine effectiveness of 33%
What about kids?
“…among children aged 6 months through 8 years, the interim estimates against any influenza and A(H3N2) virus infection were higher; the risk for A(H3N2) associated medically-attended influenza illness was reduced by more than half (59%) among vaccinated children. Also, with interim VE estimates of 67% and 42% against influenza A(H1N1)pdm09 and B viruses, respectively, vaccination provided substantial protection against circulating A(H1N1)pdm09 viruses, as well as moderate protection against influenza B viruses predominantly belonging to the B/Yamagata lineage, the second influenza type B component included in quadrivalent vaccines.”
CDC on Interim Estimates of 2017–18 Seasonal Influenza Vaccine Effectiveness — United States, February 2018
So the flu vaccine ended up being a lot more effective than folks predicted, especially in younger, higher risk kids, although it is still far from perfect.
Unfortunately, this year’s flu vaccine was not effective at preventing medically-attended influenza illness (flu case that sends you to see a doctor) for the 9 to 17 year old age group for some reason. That’s still not a good reason to skip the flu vaccine if your child hasn’t had it though, as the flu vaccine might still work to help your child, even if doesn’t fully prevent a case of the flu.
The Flu Vaccine Works
- 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
- CDC – Interim Estimates of 2017–18 Seasonal Influenza Vaccine Effectiveness — United States, February 2018
- CDC – Vaccine Effectiveness – How Well Does the Flu Vaccine Work?
- CDC – Frequently Asked Flu Questions 2017-2018 Influenza Season
- 2017 flu vaccine effectiveness – getting the facts straight
- Flu Vaccine Benefits Go Beyond Effectiveness of One Strain
- Why Get A Flu Shot?
- Ask the Experts about the Flu Vaccine
- CDC – Update: Influenza Activity — United States, October 1–November 25, 2017
- WHO – Influenza updates
- CDC – Influenza Prevention: Information for Travelers
- Study – Low interim influenza vaccine effectiveness, Australia, 1 May to 24 September 2017
- Study – Late season interim estimates of influenza vaccine effectiveness reliably predict end of season estimates in Victoria, Australia, 2007 to 2012.
- CDC – Seasonal Influenza Vaccine Effectiveness, 2005-2017
- Study – Effectiveness of inactivated influenza vaccines varied substantially with antigenic match from the 2004-2005 season to the 2006-2007 season.
- Study – Influenza vaccine effectiveness in Australia: results from the Australian Sentinel Practices Research Network.
- Study – Understanding influenza vaccine protection in the community: an assessment of the 2013 influenza season in Victoria, Australia.
- Study – Influenza vaccine effectiveness against hospitalisation with influenza in adults in Australia in 2014.
- Study – 2014–2015 Influenza Vaccine Effectiveness in the United States by Vaccine Type
- CDC – Interim Estimates of 2016–17 Seasonal Influenza Vaccine Effectiveness — United States, February 2017
- Study – Chasing Seasonal Influenza — The Need for a Universal Influenza Vaccine
- WHO – Vaccine effectiveness estimates for seasonal influenza vaccines
- No, the CDC did not just “admit” that this year’s flu vaccine doesn’t work
- Flu vaccine research and development – eggs cause low effectiveness
- 10 Things To Know About 2017-2018 Flu Shots
- If I Had Gotten the Flu Shot