Flu Shots This Year: A Good Match to the Virus?

This winter is off to a very busy start at urgent care clinics and hospitals. The CDC created a new color for “very high” influenza-like-illness (ILI) activity as nearly all states had reached a “very high” or “high” level of illness by early December. Does it still make sense to get a flu shot? How well does this year’s vaccine match the circulating strains?

Key takeaways:
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    Influenza like illness visits are higher than usual at this time of year.
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    Influenza hospitalizations are higher than they have been since the 2010-11 season.
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    Flu vaccination can reduce the severity of illness by 40-60% when the vaccine is a good match to the circulating strains.
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    The 2022 flu vaccine is a good match, and vaccination is recommended for all people ages 6 months and older.

Weekly hospital admissions for influenza (flu) are higher at this time of year than a typical flu season. By week 47, the cumulative flu hospitalization rate per 100,000 population is between 0.1 to 2.0, but this season it has already reached 16.6 as of November 26, 2022.

What strains are circulating?

By far the most common strain of flu being tested is type A (96.2%) whereas type B was only 2%. The most common flu A strain was A(H3N2) followed by A(H1N1). Of the hospitalizations with more complete laboratory data available, SARS-CoV-2 was also found in 4.32% of cases.

Who is most at risk of hospitalization due to influenza?

Both age and underlying health status affect one’s risk, similar to COVID-19.

The highest hospitalization rates per 100,000 population are among those ages 65 years or older (39.9 admissions per 100,000 population), and in this group over 65, rates were much higher among those 85 or older (71.3 per 100,000). Among younger people, hospitalization rates were higher for babies and toddlers 0-4 years of age (28.4).

Most hospitalized patients had at least one underlying condition. In adults, 96.7% had at least one medical condition, most commonly hypertension, cardiovascular disease, metabolic disorder (diabetes), chronic lung disease, or obesity. Among children, nearly 3 in 4 had at least one medical condition, the most common of which was asthma.

How well does this year’s vaccine protect against the flu?

The CDC has analyzed the genetic composition of 788 influenza virus specimens collected since May 2022 to determine how similar the circulating strains are to the vaccine strains. The viruses are also characterized antigenically by how well antibodies block or neutralize the virus. If differences in antibody levels remain below a specific threshold, the vaccine is considered to be a good match to the circulating strains. So far, the CDC reports that there is a 96% to 97% match for influenza A viruses and the single influenza B virus analyzed was also well recognized.

How well do antiviral medications work against the strains circulating right now?

All the viruses collected since October 2022 have no evidence of reduced inhibition by antivirals. This means that the antiviral medications are working as expected.

Three flu vaccines are recommended for people age 65 and older: Fluzone High-Dose Quadrivalent, Flublok Quadrivalent recombinant, and Fluad Quadrivalent adjuvanted. These vaccines were recommended in a unanimous vote by CDC’s Advisory Committee on Immunization Practices (ACIP) in June 2022. They are adjuvanted, making them a bit more robust in their ability to get the attention of the immune system. This is important for people who are more advanced in years when the immune system may be entering a period of senescence, or a less vigorous response to antigens.

There is no preference for a specific flu vaccine among people younger than 65.

Who should get vaccinated?

The CDC recommends that everyone over 6 months of age be vaccinated.

How well does the vaccine work to protect against hospitalization with flu?

The flu shot may not prevent symptoms of the flu entirely, but it does help reduce the severity of the disease. During a season when the vaccine is well matched to the circulating strains, as it is this year, the CDC estimates that flu vaccination reduces the risk of illness requiring a doctor visit by 40-60%.

The flu vaccine also reduces the risk of serious illness. A study published in 2021 found that among people hospitalized for the flu, those who were vaccinated had a 26% lower risk of needing to be admitted to the intensive care unit (ICU) and a 31% lower risk of death. Another study from 2018 found similar results: compared to unvaccinated individuals, those who had been vaccinated against influenza were 59% less likely to be admitted to the ICU and on average required four fewer days in the hospital.

Studies have shown similar protection among children and adolescents. A study conducted in 2022 found that vaccination can reduce the risk of life-threatening influenza by 75% and another study published in Pediatrics was the first to demonstrate a mortality reduction in children who were vaccinated. This study, which looked at four years of flu data, found that the vaccine reduced the risk of death by 51% among children with underlying medical conditions and by 65% among healthy children.

When should I get vaccinated?

It takes about two weeks to build up immunity, so if you are planning holiday travel or festive gatherings, there is no better time than now to get your shot. The vaccine is most protective in the first month after vaccination.

With any luck, timing your shot now will help provide you maximal protection just as the flu season is peaking. However, virus circulation may continue to be high well into the New Year. It is very hard to predict this flu season given the disruptions to normal virus circulation during the pandemic.

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