The Guardian reported that some Dutch vaccination centers are offering free pickled herring as an incentive to get the vaccine. June 15 is usually Vlaggetjesdag (Dutch Wikipedia), translated as “little flag day” because fishing ships would be decorated with flags on the Saturday before Pentecost. It is the day that the season’s first barrel of herring is auctioned off for a good cause and the day that herring starts appearing in supermarkets. Last year the first barrel was given to medical workers and this year the first barrel was given to the GGD health service; GGD is responsible for Covid-19 vaccinations in the Netherlands.
If you were lucky enough to be around IJmuiden you could also receive free herring after your vaccination (article from Haarlems Dagblad, but it’s behind a paywall).
In case you are wondering: I would politely decline if I was offered any. But for some Dutchies the first herring of the year can be a big thing.