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Twice each year international influenza experts meet to discuss recent developments in human and animal influenza viruses around the world, and to decide on the composition of the next influenza season’s flu vaccine. Due to the time it takes to manufacture and distribute a vaccine, decisions on which strains to include must be made six months in advance.
Which means the composition of the northern hemisphere’s vaccine must be decided upon in February of each year, while decisions on the southern hemisphere’ vaccine are made in September.
Even during normal times this is a formidable task, but over the last 30 months it has been complicated by the COVID pandemic, which has greatly suppressed global flu activity and forced many countries to put influenza surveillance on the back burner.
In 2021, we saw between a 50-to-100-fold reduction in influenza submissions to reference laboratories (see ECDC: Influenza Virus Characterisation - July 2021), and while those number have substantially improved in 2022, some gaps remain. The WHO explains in their FAQ.
What impact has the COVID-19 pandemic had on GISRS influenza surveillance? Influenza surveillance was disrupted during the early stages of the COVID-19 pandemic but has since recovered in most countries. In the 2019-2020 influenza season, over four million clinical specimens worldwide were tested and reported to WHO, and more than 8000 representative influenza viruses were shared with the WHO CCs for further analyses.While more clinical specimens were tested globally during 2020 and 2021, there were reduced numbers of influenza detections and consequently fewer viruses were available to be shared. In 2022, influenza virus detections have returned to pre-COVID-19 pandemic levels in many countries in the southern hemisphere. A(H3N2), A(H1N1)pdm09, and B/Victoria-lineage influenza B viruses have been detected in several countries. A sufficient number of influenza viruses have been available for characterization to support WHO recommendations for vaccine composition for the southern hemisphere.
Today the WHO has announced their recommendations for the 2023 Southern Hemisphere's flu vaccine, which keeps 3 of the 4 strains included in this year's Northern & Southern Hemisphere's vaccines, but replaces the 2019 H1N1 strain for a newer A/Sydney/5/2021 (H1N1)pdm09-like virus.
Recommended composition of influenza virus vaccines for use in the 2023 southern hemisphere influenza season
23 September 2022 | Meeting report
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Overview
It is recommended that quadrivalent vaccines for use in the 2023 southern hemisphere influenza season contain the following:
Egg-based vaccines
• an A/Sydney/5/2021 (H1N1)pdm09-like virus;
• an A/Darwin/9/2021 (H3N2)-like virus;
• a B/Austria/1359417/2021 (B/Victoria lineage)-like virus; and
• a B/Phuket/3073/2013 (B/Yamagata lineage)-like virus.
Cell culture- or recombinant-based vaccines
• an A/Sydney/5/2021 (H1N1)pdm09-like virus;• an A/Darwin/6/2021 (H3N2)-like virus;• a B/Austria/1359417/2021 (B/Victoria lineage)-like virus; and
• a B/Phuket/3073/2013 (B/Yamagata lineage)-like virus.
It is recommended that trivalent vaccines for use in the 2023 southern hemisphere influenza season contain the following:
Egg-based vaccines
• an A/Sydney/5/2021 (H1N1)pdm09-like virus;
• an A/Darwin/9/2021 (H3N2)-like virus; and
• a B/Austria/1359417/2021 (B/Victoria lineage)-like virus.
Cell culture- or recombinant-based vaccines
• an A/Sydney/5/2021 (H1N1)pdm09-like virus;
• an A/Darwin/6/2021 (H3N2)-like virus; and
• a B/Austria/1359417/2021 (B/Victoria lineage)-like virus
We never know how good of a match each year's flu vaccine will end up being for circulating strains until at least half-way through the flu season. For the Northern Hemisphere, that means January or February.
While flu vaccines are far from perfect, and can’t promise 100% protection, getting vaccinated – along with practicing good flu hygiene (washing hands, covering coughs and/or wearing a mask & staying home if sick) – remains your best strategy for avoiding the flu and staying healthy this winter.But it is somewhat comforting that the WHO has enough confidence to retain the H3N2 component in this fall's vaccine in next year's Southern Hemisphere vaccine.
And given some of the early warning signs suggesting a difficult flu season ahead (see South Korean CDC Issues Unusual Early Seasonal Flu Warning), is worth getting now, before flu season ramps up.
