Twice a year, 40 scientists gather together for five days to decide what strains of influenza to vaccinate against for the next flu season. It takes around six months to prepare the vaccine – which usually includes protection against three different strains of flu.

Europe and the US are heading into a flu season that some are warning could be particularly severe this winter. While even as summer approaches in Australia, the country is still registering high numbers of cases after a record-breaking flu season earlier in the year.

So how does the process of deciding on a flu vaccine each year actually work? And does what happens in the southern hemisphere influence the way the virus circulates in the northern hemisphere?

In this episode, we speak to Ian Barr, deputy director for the WHO Collaborating Centre for Reference and Research on Influenza, based at the Peter Doherty Institute for Infection and Immunity, part of the University of Melbourne. Barr is one of those 40 scientists who attend the meetings to decide what strains to focus vaccination efforts on.

This episode was produced by Mend Mariwany, Katie Flood and Gemma Ware. Mixing by Michelle Macklem and theme music by Neeta Sarl. Read the full credits for this episode and sign up here for a free daily newsletter from The Conversation.

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