Is the Flu Vaccine Effective?

Check out this answer from Consensus:

Influenza vaccines are generally effective in reducing the incidence of influenza, influenza-like illness, and related complications across various age groups and settings. The effectiveness is particularly notable in children and high-risk groups, though it can vary significantly by season and strain. In elderly populations, the vaccine’s effectiveness is more pronounced in preventing severe outcomes like hospitalization and mortality, especially in long-term care facilities. Continued monitoring and development of more effective vaccines are essential to enhance protection against influenza.

The effectiveness of the influenza vaccine is a critical public health concern, especially given the annual variability in influenza virus strains and the significant morbidity and mortality associated with influenza infections. This synthesis examines the efficacy and effectiveness of influenza vaccines across different age groups and settings based on multiple research studies.

Key Insights

• General Effectiveness in Adults and Children:
  • Influenza vaccines provide moderate protection against virologically confirmed influenza, with trivalent inactivated vaccine (TIV) showing a pooled efficacy of 59% in adults aged 18-65 years and live attenuated influenza vaccine (LAIV) showing a pooled efficacy of 83% in children aged 6 months to 7 years1.
  • The effectiveness of influenza vaccines against medically attended influenza in outpatient or inpatient settings varies, with significant protection observed in some studies1.
• Effectiveness in Elderly Populations:
  • Influenza vaccines are effective in reducing influenza-like illness by 35%, hospitalization for pneumonia and influenza by 33%, and mortality following hospitalization for pneumonia and influenza by 47% in individuals aged 65 years and over living in the community2.
  • In elderly individuals, vaccines prevent pneumonia (46%), hospital admission (45%), and deaths from influenza or pneumonia (42%) in long-term care facilities, though effectiveness in the community is modest4.
  • Influenza vaccines provide moderate protection against influenza-associated hospitalizations among adults, with lower protection observed in elderly during seasons with antigenically variant strains5.
• Effectiveness in High-Risk Groups:
  • Influenza vaccination is effective in preventing infection, hospitalization, and mortality among high-risk groups, including children, elderly, and individuals with underlying health conditions10.
  • In children, influenza vaccination offers significant protection against influenza-related hospitalization, with higher effectiveness in fully vaccinated children9.
• Effectiveness Against Specific Influenza Strains:
  • The effectiveness of the influenza vaccine against the 2009 pandemic influenza A(H1N1) virus was 54% overall, with significant residual protection observed in individuals who had received prior A(H1N1)pdm09-containing vaccines6.
  • During the 2016-2017 season, the influenza vaccine provided significant protection against medically attended influenza due to A/H3N2 and B viruses, with an overall adjusted vaccine effectiveness of 43%7.

 

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Is the Flu Vaccine effective?

Jeff Kwong has answered Near Certain

An expert from University of Toronto in Epidemiology, Medicine

First, let’s be clear that influenza vaccines are only designed to prevent illness due to influenza viruses. Many other viruses (e.g., RSV, parainfluenza, adenovirus, etc.) circulate each fall and winter that influenza vaccines are not designed to protect against, and it is impossible to distinguish between the various viruses without a lab test. Second, influenza vaccines DO reduce the risk of influenza infection at least a bit for most years. The level of protection depends on the strains that are circulating and the strains contained in the vaccine. Influenza vaccines contain 3 or 4 different strains: 1 H1N1 strain, 1 H3N2 strain, and 1 or 2 B strains. The predominant strain(s) vary each year, but often we see 1 H1N1 strain and 1 B strain dominate, or 1 H3N2 strain and 1 B strain dominate. When you read stories about how well the influenza vaccine worked, you will often see that it offered X% protection against the H3N2 strain and Y% protection against the B strain. So just because effectiveness is low against 1 strain, the vaccine is still worth getting due to greater protection against another strain. For example, US data for the 2017-18 season (https://www.cdc.gov/mmwr/volumes/67/wr/mm6706a1.htm?s_cid=mm6706a1_w) showed vaccine effectiveness of 25% (95% confidence interval [CI] = 13%–36%) against H3N2, which is on the lower end, but 67% (95% CI = 54%–76%) against H1N1, and 42% (95% CI = 25%–56%) against influenza B, which are obviously more desirable. The 2017-18 season (so far) has been characterized by a lot of H3N2 and B activity, so getting the influenza vaccine has been worth it for the protection provided against influenza B even if the protection provided against H3N2 has been on the lower side. Meta-analyses (studies that combine data from previously published studies) have shown that influenza vaccines do offer some protection (higher against H1N1 and B than H3N2) most years (see Belongia EA, et al. Lancet ID 2016. Variable influenza vaccine effectiveness by subtype: a systematic review and meta-analysis of test-negative design studies). The problem is that we cannot predict at the start of the influenza season how well the vaccine will work, so the best bet is to get it, especially if you are at high risk of serious complications from influenza infection. Last, I think we should look at the effectiveness provided by influenza vaccines a little differently. We expect all medical interventions to be 100%, but the reality is that few interventions are that effective. Yes, some childhood vaccines, the HPV vaccine, and the new shingles vaccine (Shingrix) are 95+% effective. But many drugs provide much lower levels of protection than people think. For example, for someone who has had a heart attack, statins (medications that lower cholesterol) are highly recommended for preventing another heart attack, but they are “only” 25% (95% CI = 19%–30%) effective in preventing fatal or non-fatal cardiovascular disease (from the Cochrane Review on that topic). Yes, having a heart attack (whether fatal or not) is far worse than getting influenza infection, but sometimes influenza can cause serious illnesses as well, and people can end up in the hospital or dying as a result of influenza infection. So if you can reduce the chances of bad things resulting from influenza happening by preventing influenza infection in the first place, even if the vaccine is “only” 25% effective, wouldn’t that be a good idea?

 

Is the Flu Vaccine effective?

Ryan Malosh has answered Likely

An expert from University of Michigan in Epidemiology

The short answer is yes, but not as effective as we want it to be.

A couple of distinctions in how we study whether or not influenza vaccines work.

1) Vaccine effectiveness studies examine how well the vaccine works in the real world while vaccine efficacy studies examine how well the vaccine works in clinical trials. Influenza vaccine efficacy studies are no longer ethical in the US because the CDC recommends annual vaccination for all persons >6 months of age.

2) Vaccine effectiveness (VE) can be measured against a wide range of outcomes including infection, infection and severe disease, transmission between contacts, etc. The most common studies of influenza VE measure how well the vaccine works at preventing influenza infections that lead to a doctor visit. Influenza VE measured this way varies by year and which viruses were most common. A recent meta-analysis showed that VE tends to be better against influenza B (61%) and influenza A/H1N1 (54%) than against influenza A/H3N2 (33%). Many studies are currently being done to figure out why.

My research currently involves studies to look at how well the vaccine prevents severe infections and how well high levels of vaccine coverage in small communities (e.g. households) can protect unvaccinated individuals.

 

Is the Flu Vaccine effective?

Chuanxi Fu has answered Likely

An expert from Zhejiang Chinese Medical University in Epidemiology

The seasonal flu vaccine was designed to prevent certain diseases infected by three or four combined virus strains – usually A(H1N1), A(H3N2) and B (one or two strains). Besides the direct protection on the vaccinees, receiving the flu vaccine will probably also benefit the relatives, friends and colleagues (ie., indirect protection effect).

Many factors could impair the flue vaccine effectiveness in filed ,which was different from the efficacy in the clinical trails), including: i) the accuracy of prediction for future epidemic strains in the whole world, ie, match or mismatch between the circulating strains and vaccine strains

1. ii) the production technology

iii) the status of vaccinees, for example, the elderly will benefit less than the youth

To the extent, the flu vaccine can protect 60%+ of vaccines from flu infection. In fact, the effectiveness varies a lot, based on our prior data (2010-2016), 0% – 70%, which was verified by other similar studies internationally. We also observed the residual protection effect by flu vaccination years ago. Although the limited benefit brought by flu shot, the vaccination is the most effective weapon to prevent the flu. Receiving vaccination should be encouraged for the risk groups.

 

Is the Flu Vaccine effective?

Sheena Sullivan has answered Likely

An expert from University of California, Los Angeles in Epidemiology

This question “Is the flu vaccine effective” is a little too broad to answer easily. It depends on what your outcome of interest is (e.g. infections, hospitalisations, deaths), which virus you are concerned with (i.e. A(H3N2), A(H1N1)pdm09, B/Yamagata, B/Victoria), your personal threshold of what constitutes “effective” (e.g. 20%, 50% 80%), whether you expect it to be effective every year or just most, whether you expect it to be equally effective for all members of a population, and many other caveats. In general, the vaccine will reduce your risk of infection (as measured by a GP visit or hospitalisation). However, the degree to which it will reduce infection is not constant and will vary by subtype and lineage, your prior exposure to the vaccine, your influenza infection history, the degree of antigenic match between the vaccine strains and circulating strains, your age, whether you have any other conditions that increase your risk of a severe outcome and probably some other factors.

 

Is the Flu Vaccine effective?

Michael Haber has answered Likely

An expert from Emory University in Biostatistics

The effectiveness of the influenza vaccine varies from year to year as the strains (types) of the influenza virus keep changing. Even when the strains included in the vaccine match the actual strains, the effectiveness is only around 60%. This means that if you receive the vaccine than your chances of getting flu are at least 40% of what they were without vaccination. The good news is that vaccinated persons who develop influenza despite being vaccinated usually have milder symptoms. Therefore CDC and most physicians recommend that everyone (except for young babies and people with certain health issues) becomes vaccinated every year.

 

Is the Flu Vaccine effective?

Rebecca Jane Cox Brokstad has answered Likely

An expert from University of Bergen in Immunology

Influenza vaccines are moderately effective vaccine in preventing the serious complication of influenza such as hopsitilsation and death(http://www.thelancet.com/journals/laninf/article/PIIS1473-3099(11)70295-X/abstract). The World Health organisation recommends updating of the composition of vaccine strains, which contain influenza A/H1N1 and A/H3N2, together with the dominant influenza B lineage in circulation (trivalent) or both B lineages (quadrivalent) twice a year (once for Northern Hemisphere and once for Southern Hemisphere). However as the virus rapidly evolves in some years the vaccine strains do not always provide protection against new epidemic strains. 

As examples, the late emergence of antigenic variant H3N2 circulating strains led to the mismatch and low seasonal influenza vaccine  effectiveness in season 1997-98, 2014-15 and in the current season 2017/18.

https://www.cdc.gov/mmwr/volumes/67/wr/mm6706a2.htm, http://www.eurosurveillance.org/content/10.2807/1560-7917.ES.2018.23.7.18-00057#html_fulltext

http://www.eurosurveillance.org/content/10.2807/1560-7917.ES.2018.23.5.18-00035

 

Is the Flu Vaccine effective?

Angeliki Melidou has answered Likely

An expert from Aristotle University of Thessaloniki in Microbiology, Virology

Vaccine effectiveness varies depending on the antigenic characteristics of the viruses that circulate each influenza season and how good we have predicted them and included them in the vaccine. As more than one type (of the two influenza A and the two influenza B viruses that circulate in humans) of influenza viruses circulate each season, the vaccine most likely will be effective for at least one or two of them, more than the others. But even though for one type it could be for some seasons only as low as 10% effective, this still would mean that 10 out of 100 people will be saved!Considering that a billion infections happen each year throughout the world, even this percentage is important!Vaccination is the only way to prevent influenza infections and is essential to vaccinate, especially people that belong to high risk groups!

 

Is the Flu Vaccine effective?

Mark Doherty has answered Near Certain

An expert from GlaxoSmithKline in Immunology, Medicine

If we define “effective” as “reducing the chance of illness or death from flu”, then yes, the flu vaccine is effective.

The Cochrane review team (1-3), who routinely review all published studies in this area, found that clinical trials in more than 300,000 people – from different areas, exposed to different strains of flu have shown that on average, the standard, injected, non-living flu vaccine reduces the chance of getting influenza by 78% in children, with slightly lower protection in adults or older people. Essentially, that means on average the vaccine cuts your risk of getting ill from influenza about 4-fold to 5-fold. But there are some catches.

Although we talk about “the flu” in reality there are thousands of different strains of flu circulating at any one time. Many of these are rare, and cause only a little disease, but the commonest ones are associated with the flu epidemics we see every winter. In most years during flu season, the epidemic is made up of many different strains, but the bulk of the disease will be caused by a small number of common strains, and what’s common will change from year to year.

So if you get a natural infection with influenza, you develop some immunity – but only to the specific strain that infected you, and maybe to a lesser extent, closely-related strains. That’s why even if you have had influenza, you can get it again, and again: because being immune to one strain of virus that is common this winter won’t necessarily protect you against all the other stains of virus that are around.

The vaccine works the same way as a natural infection – typically the vaccine will give a very high degree of protection against the specific strain that the vaccine was made against, and perhaps a little protection against closely related strains. But current vaccine technology cannot make a vaccine that will protect you against all strains of flu. That’s why you need a new vaccination every year – to cope with the changing influenza virus population.

So what happens is that public health workers all around the globe continuously monitor influenza disease all around the world, tracking which strains are most common, which are spreading fastest, and which ones are causing the most disease. Based on that, they share their data, and try to predict what influenza strains will be the biggest problem in the coming winter. Companies develop their vaccines based on that – vaccines typically protect against the 3 or 4 strains of influenza predicted to be most common. If the prediction is accurate, we can expect good protection, because most of the circulating virus will match the vaccine. But if the prediction is inaccurate – perhaps because the virus mutated, or because one strain starts spreading much faster than expected – then we get a poor match with the vaccine, and overall protection will be lower.

References

1. Demicheli V. Cochrane Database Syst Rev. 2018 Feb 1;2:CD004879. doi: 10.1002/14651858.
2. Vaccines for preventing influenza in healthy adults. Demicheli V, Jefferson T, Ferroni E, Rivetti A, Di Pietrantonj C. Cochrane Database Syst Rev. 2018 Feb 1;2:CD001269. doi: 10.1002/14651858.
3. Vaccines for preventing influenza in the elderly. Demicheli V, Jefferson T, Di Pietrantonj C, Ferroni E, Thorning S, Thomas RE, Rivetti A. Cochrane Database Syst Rev. 2018 Feb 1;2:CD004876. doi: 10.1002/14651858.CD004876.