Tag: vaccine strains

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 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.

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.

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

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

Measles Vaccines vs Measles Strains

Most people understand that for every virus or bacteria, their can be multiple strains of the same organism that cause disease.

For example, there is flu A and B, swine flu, bird flu, and even dog flu.

In the case of flu, those different strains are a problem, because having immunity to one, doesn’t mean that you will have immunity to others. In fact, usually you won’t, whether it is natural immunity from a previous infection or immunity from a vaccine.

Pains with Strains

Do we have the same issues with other diseases?

We certainly have situations in which vaccines don’t cover all disease strains, including:

  • Gardasil – now covers nine strains of HPV that cause 90% of cervical cancers
  • Hib – only covers Haemophilus influenzae type b, which causes invasive disease, like meningitis, pneumonia, and sepsis, but not other Haemophilus influenzae strains that can cause ear infections
  • Polio – originally protected against three serotypes of polio, but monovalent (one strain) and bivalent (two strains) oral poliovirus vaccines have also been available to respond to outbreaks and bOPV is the one used for routine immunization, except in industrialized, polio-free countries that use the IPV shot
  • Prevnar – now covers 13 strains of Streptococcus pneumonia
  • Rotavirus – protects against severe disease caused by rotavirus strains that aren’t even in the vaccine

Fortunately, even when a vaccine doesn’t cover all strains, it does cover those that most commonly cause disease.

Measles Genotypes

Knowing the genotype of a measles strain can help you understand where measles outbreaks are coming from.
Knowing the genotype of a measles strain can help you understand where measles outbreaks are coming from.

What about measles?

There are at least 24 different genotypes of measles that come from 8 different clades (A-H), with even more wild type virus strains (based on those genotype).

These genotypes include A (all vaccine strains are genotype A), B2, B3, C1, C2, D2, D3, D4, D5, D6, D7, D8, D9, D10, D11, G2, G3, H1, and H2.

In general, genotypes are restricted to a specific part of the world, such as:

  • African Region – B2, B3
  • Eastern Mediterranean Region – B3, D4, D8
  • European Region – D4, D5, D6
  • Southeast Asian Region – D4, D5, D8, D9, G2, G3, H1
  • Western Pacific Region – D5, D9, G3, H1

In countries that have eliminated measles, like the United States, the genotypes that are found will depend on from where the measles strain was imported.

Additionally, five genotypes, B1, D1, E, F, and G1 are now inactive.

Measles Strains

Specific strains of measles viruses include the vaccine strains (Edmonston, Moraten, Zagreb, Schwarz, AIK‐C, CAM, Leningrad-16, and Shanghai-191, etc.) plus wild strains, like:

  • MVi/NewYork.USA/94 – a wild strain of B3 genotype
  • Johannesburg.SOA/88/1 – a wild strain of D2 genotype
  • Manchester.UNK/30.94  – a wild strain of D8 genotype
  • Hunan.CHN/93/7 – a wild strain of H1 genotype

Why so many vaccine strains?

It may come as a surprise to some people, but the whole world doesn’t use the same vaccines. For example, unlike the United States, Japan has used measles vaccines derived from AIK‐C, CAM, and Schwarz strains of the measles virus.

And just how many wild strains of measles are there? It’s hard to know, but consider that a study of 526 suspected measles cases from 15 outbreaks over 3 years in one state of India found at least 38 different strains.

Myths About Measles Strains

Do the measles vaccines cover all of the measles strains that cause outbreaks around the world?

Yes they do, despite the myths you may hear about mutated measles strains.

This came up a lot during the Disneyland measles outbreak, when folks first tried to place blame on a vaccine strain and then on the fact that the outbreak strain didn’t match the vaccine strain.

“…California patients were genotyped; all were measles genotype B3, which has caused a large outbreak recently in the Philippines…”

CDC Measles Outbreak — California, Dec 2014–Feb 2015

And it is coming again in the latest measles outbreak in Minnesota. Could that outbreak be caused by a vaccine strain? Anything is possible, but it’s not. A communication’s director for the Minnesota Department of Health has confirmed that “that the virus strain making people sick in this outbreak is the B3 wild-type virus.”

Of course, none of these outbreaks are started by a vaccine strain of measles shed from someone who was recently vaccinated. It also had nothing to do with the fact that the strains didn’t match – after all we aren’t talking about the flu.

These outbreaks are imported from other countries by folks who typically aren’t vaccinated or are incompletely vaccinated and mostly spread among other people who are unvaccinated.

So what’s the most important thing to understand when considering all of these vaccine strains and wild strains of measles? It is that “there is only 1 serotype for measles, and serum samples from vaccinees neutralize viruses from a wide range of genotypes…”

In other words, the measles vaccine works against all strains of measles in all genotypes of measles. That makes sense too, because the measles virus, unlike influenza, is monotypic.

There is only one main type of measles virus, despite the many small changes in the virus that can help us identify different strains and genotypes. And these changes don’t affect how antibodies protect us against the measles virus.

What To Know About Measles Strains

The best way to get protected against all measles strains is to get vaccinated with two doses of the MMR vaccine.

More About Measles Strains

Updated May 23, 2017