Are influenza vaccinations a.k.a. flu shots the same worldwide? For example, if the patient gets a flu shot in South Korea in October, then spends the winter in the United States, would the protection against the influenza be as efficient as if the patient had gotten the flu shot in the United States?

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There's one vaccine composition for the southern hemisphere and one for the northern hemisphere each year.

The WHO publishes the composition recommendations.

For example, the composition for the 2019/2020 northern hemisphere winter season was:

  • an A/Brisbane/02/2018 (H1N1)pdm09-like virus;
  • an A/Kansas/14/2017 (H3N2)-like virus;
  • a B/Colorado/06/2017-like virus (B/Victoria/2/87 lineage); and
  • a B/Phuket/3073/2013-like virus (B/Yamagata/16/88 lineage).

But for the 2020 southern hemisphere winter season, it was:

  • an A/Brisbane/02/2018 (H1N1)pdm09-like virus;
  • an A/South Australia/34/2019 (H3N2)-like virus;
  • a B/Washington/02/2019-like (B/Victoria lineage) virus; and
  • a B/Phuket/3073/2013-like (B/Yamagata lineage) virus.

Some components are the same but some differ: if you take a northern hemisphere vaccination in October, you can expect to be less protected in the southern hemisphere flu season next year if you travel.

The effectiveness of the vaccination depends in part on how close the actual virus you're infected with matches one of the components in your vaccination. The vaccines are manufactured several months before the flu season starts.

When it does, different areas experience different levels of each kind of influenza virus.

Here's map of the dominant type of influenza virus for week 5 of 2020 in Europe:

ECDC map of dominant type (source)

ECDC also publishes how well the vaccine matches each circulating virus (example report) so it is true that you will be differently protected in different areas of the world.

However, the vaccine composition is the same, in each season, for each hemisphere. When you take flu vaccination in the United States, your vaccine has the same effect as the vaccination is South Korea. The actual vaccine may come from a different manufacturer, but the composition will be the same.

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