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. So in February, the group’s decision affects the northern hemisphere’s flu season, and in September, it’s about the southern hemisphere.

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 ofThe Conversation Weekly podcast, 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.

After a tour around his lab, Barr explains how the different parts of the global flu monitoring system cooperate – and why it can be misleading to think that what happens in the southern hemisphere influences the northern hemisphere, and vice versa. Barr says that might be the case in some years – including in 2025 – but in “other years, I think it’s less clear that the viruses are coming from south to north … they may come from other places that have had unseasonable outbreaks during the summer or autumn.”

Listen to the interview with Ian Barr on The Conversation Weekly podcast.

This episode of The Conversation Weekly was written and produced by Gemma Ware and Mend Mariwany with assistance from Katie Flood. Mixing by Michelle Macklem and theme music by Neeta Sarl.

Newsclip in this episode from 7News Australia.

Listen to The Conversation Weekly via any of the apps listed above, download it directly via our RSS feed or find out how else to listen here. A transcript of this episode is available via the Apple Podcasts or Spotify apps.

This article is republished from The Conversation, a nonprofit, independent news organization bringing you facts and trustworthy analysis to help you make sense of our complex world. It was written by: Gemma Ware, The Conversation

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Ian Barr owns shares in an influenza vaccine producing company, and his centre receives funding from commercial groups for ongoing activities.