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It's a global surveillance system. We look at past years and what strains currently circulating in the hemisphere of the earth where it's winter.

We try to match that, but it's not perfect.

Not all strains of flu work well in vaccines, some don't grow well in culture and some we just get wrong

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Recommendations for vaccines are made globally for each flu season, but it’s important to note that there are actually two “flu seasons” in a year, the northern hemisphere and the southern (corresponding to winter in each). The recommendations are made every 6 months, so other parts of the world may have a different vaccine in the same calendar year.

Since flu season corresponds to actual seasons the northern and southern hemisphere alternate flu seasons. The flu stains circulating are monitored in the opposite hemisphere and used to predict which vaccine to give in the upcoming season.

so if you spent winter in the southern hemisphere and then winter in the northern hemisphere would a doctor recommend getting both vaccines?

Depends if there has been a strain change in the vaccine for either hemisphere. Some years it remains the same for both, in which case no you could probably just have the 1.

This is actually a big question to answer because a lot of surveillance goes into it. It's not just about looking the past strains of flu which have been circulating and which strains went into the vaccines. It's also how the populations current immunity reacts to the circulating viruses in lab tests.

There are several WHO Collaborating Centres around the world which receive virus samples taken from positive flu cases sent by various National Influenza Centres globally. The virus samples are put into culture in the hope they grow, sequenced if they do and a genetic tree is made to see if there are any obvious outliers, strains to watch etc.

During this time there are calls for blood donations (usually from people who work in these places!) pre and post flu vaccination. The serum from the blood is sent to each WHO collaborating centre so they can be used to test against viruses which are circulating/deemed of interest.

All of those results are pulled together ready for the vaccine composition meeting which happens twice a year (the northern hemisphere one is happening this week, southern is normally September) and a decision is made as to which strains of Influenza A and B go into the next jab. Then the candidate viruses get grown up, and passed on to the manufacturers of the vaccines. This has a time limit, about 4-5 months before the new vaccines and reagents for testing them have to be sent to the WHO Essential Regulatory Laboratories (ERLs), to undergo strict quality control tests to ensure they meet the standards expected before being used in the public health care system.

This video actually explains it quite nicely if TLDR! Monitoring Influenza Around the World

You can also learn more about GISRS and the Global Influenza Program here https://www.who.int/teams/global-influenza-programme#

This is actually my job, I work within the GISRS network - used to be in an ERL, now a WHO CC.

Wow, thanks!

Know if they are incorporating or considering any sequences extracted from wastewater surveillance? Or strictly clinical sequences?

For flu I've not heard of any plans to do so but it's a question I can ask. They have done it for covid and polio I know that much. I would presume as the flu surveillance program is so well established using clinical samples, it might be deemed unnecessary to test waste water?

Although from my own point of view, we actually don't get the full picture of how much flu/what types of flu etc are out there globally in a season because we only get clinical samples (I say only, we get thousands of them!). How many people are actually sick with flu, but don't need medical attention, thus not getting swabbed? It is something I've brought up in general discussion and there was agreement, but currently not much we can do about it.

Have we seen any reduction in other vaccine usage since the recent big anti-vax wave? I'm wondering if there are a lot of people who used to be fine with the flu vaccine have changed their mind

I want to say thanks for your work. I have been getting mine every year for the past 10 years.

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It's what we're here for :) as long as the stuff isn't going to waste it's all good!