Uncertainty Remains in the Match or Mismatch for the 2019-2020 Flu Vaccine


Just as vaccinations get under way in full force, a few indicators raise the question about whether the vaccine really is as well matched as was anticipated.

Authorship of this article was updated on October 7, 2019.

When the CDC announced changes to the 2019-20 influenza vaccine in the spring of 2019, it was with the anticipation that the vaccine would be a much better match to the influenza virus than it has been in the past few seasons.1

Vaccines for the 2019-2020 Northern Hemisphere saw updates to both the H3N2 and H1N1 influenza A strains, which had not been updated in a couple of years.1 Additionally, all regular-dose and recombinant vaccines are now quadrivalent and therefore covering both lineages of influenza B.1 The CDC still continues to recommend that vaccination be offered by the end of October but added caution against vaccinating before September, as it could potentially result in reduced protection later in the influenza season.1 Current influenza activity is below the national baseline of 2.2%, but influenza prevalence can be unpredictable and the influenza season in the Northern Hemisphere is only just starting.

The recommendations for the CDC 2019-2020 quadrivalent influenza vaccine are:

  • A/Kansas/14/2017 (H3N2)-like virus
  • A/Brisbane/02/2018 (H1N1)pdm09-like virus
  • B/Colorado/06/2017-like virus (Victoria lineage)
  • B/Phuket/3073/2013-like virus (Yamagata lineage)

However, just as vaccinations get under way in full force, a few indicators raise the question about if the vaccine really is as well matched as was anticipated. The influenza season in the Southern Hemisphere generally lasts from April to September and precedes the northern hemisphere’s season. As a result, trends in vaccine effectiveness and influenza infection in the Southern Hemisphere may give clinicians in the Northern Hemisphere potential clues about what to expect.

A study published by Gouma et al. indicates that there was an antigenic mismatch of the 2019 Southern Hemisphere influenza vaccine within the H3N2 component.3 The H3N2 strain A/Switzerland/8060/2017 was recommended for their seasonal vaccine in 2019. Unfortunately, the strain was ultimately found to have a substitution that results in the loss of a glycosylation site in the antigenic site B of hemagglutinin which is associated with low vaccine effectiveness during H3N2-dominated influenza seasons.3

The World Health Organization (WHO) advisory group announced in September 2019, its recommendations for the 2020 Southern Hemisphere influenza vaccine composition, and 2 of the 4 recommended components are different than the vaccines now being administered to patients in the Northern Hemisphere.4 Although the H1N1 and influenza B Yamagata lineage components mirror what is in the 2019-20 Northern Hemisphere vaccine, the WHO recommends the A/south Australia/34/2019 (H3N2)-like virus and the B/Colorado/06/2017-like virus (Victoria lineage).4

Although it is impossible to definitively know what activity will be like for any given influenza season, the new WHO recommendations may be a reason for pause. Influenza A, particularly H3N3, have been the cause of the majority of influenza cases in the Southern Hemisphere in 2019.5 Likewise, while most influenza B lineages were not determined, when it was, the Victoria linage was much more prevalent than the Yamagata lineage.5 Epidemiologists in both hemispheres monitor influenza activity worldwide when determining what components to recommend. With the season just getting under way in the Northern Hemisphere, it seems premature to panic about the match an effectiveness of our vaccine. However, clinicians should be aware that the possibility of a vaccine mismatch and that a season with high influenza activity might be upon us again this year.

Truthfully, it is too soon to know what will be the northern hemisphere’s dominant circulating influenza. It may end up that the epidemiologists’ recommendations for our vaccine composition was well matched after all. It is also important to remember that while WHO recommended different H3N2 and Victoria lineage components, the H1N1 and Yamagata lineage components are the same as what is and will be administered to patients in the northern hemisphere over the next few months. Although 1 influenza virus traditionally dominates in a season, the other viruses can and do cause infections in respectable amounts, as seen with the data from the southern hemisphere.5

There can be frustration among patients when the vaccine is mismatched, especially when it appears to occur in consecutive years and may provoke doubt or hesitancy about obtaining vaccination. However, it is important for pharmacists to counsel patients that vaccination will not be without benefit. Even if 1 of the strains is mismatched, the influenza vaccine can still provide immunity to other influenza viruses and may mitigate the severity of illness if a patient does become infected. Therefore, the influenza vaccine should still be recommended as a critical component of preventable health and wellness.


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  • CDC. Frequently Asked Influenza (Flu) Questions” 2019-2020 Season. Availabile: https://www.cdc.gov/flu/season/faq-flu-season-2019-2020.htm?CDC_AA_refVal=https%3A%2F%2Fwww.cdc.gov%2Fflu%2Fseason%2Fflu-season-2019-2020.htm (Accessed 3 October 2019)
  • Grohskopf LA, Alyanak E, Broder KR, Walter EB, Fry AM, Jernigan DB. Prevention and Control of Seasonal Influenza with Vaccines: Recommendations of the Advisory Committee on Immunization Practices — United States, 2019—20 Influenza Season. MMWR Recomm Rep 2019;68(No. RR-3):1–21.
  • Gouma S, Weirick M, Hensley SE. Potential antigenic mismatch of the H3N2 component of the 2019 Southern Hemisphere influenza vaccine. Clinical Infectious Diseases. Doi:10.1093/cid/ciz723
  • World Health Organization. Recommended composition of influenza virus vaccines for use in the southern hemisphere influenza season. Available: https://www.who.int/influenza/vaccines/virus/recommendations/2020_south/en/ (updated 27 September 2019; accessed 3 October 2019)
  • World Health Organization. Influenza Laboratory Surveillance Information. Available: https://apps.who.int/flumart/Default?ReportNo=5&Hemisphere=Southern (accessed 3 October 2019)

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