Flu vaccines take months to make. Here's what could speed it up

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For decades, the flu virus in the shots you’re offered every fall has been grown in chicken eggs. 

While that may sound odd, the tried-and-true technology has been around since the 1940s. Now, scientists are trying more modern methods of developing vaccines.

It’s not a diss against the millions of hens and their eggs that go into making influenza vaccines for the world every year, because the system does work, experts say. But supplies can run short, and eggs can cause problems that researchers are making progress on solving.

Twice a year, the World Health Organization convenes an expert panel to decide what should go into vaccines for the coming flu season in each hemisphere, based on the highest-circulating strains. It takes six months to decide which strains to include, purify the seed ingredients, mass produce the shots in eggs and get them into vials to go into arms. 

The steps add up to a sluggish ability of vaccine makers to adapt to the fast-evolving virus, meaning last year’s vaccine may not protect against this year’s strain. And the respiratory disease is highly contagious, in most cases causing fever, coughing and body aches, but potentially leading to severe complications like pneumonia.

Another part of the problem is that many flu viruses originated in birds, virologists say. 

"You're growing it in embryonated chicken eggs," said Dr. Lynora Saxinger, an infectious diseases specialist at the University of Alberta in Edmonton. "And so that's avian cells that are growing the virus for you and the virus actually adapts to grow better in the avian cells."

That can sometimes result in viruses that look less like what's causing infections in humans, which can further decrease the effectiveness of influenza vaccines, said Matthew Miller, director of the Degroote Institute for Infectious Disease Research at McMaster University in Hamilton, Ont.

Last week, the New England Journal of Medicine published Phase 3 of a clinical trial pointing to a potentially more effective option: mRNA flu shots.

Pfizer scientists tested a flu shot made with the same mRNA technology used in COVID-19 vaccines against a traditional egg-based flu vaccine in more than 18,000 adults in the U.S., South Africa and the Philippines during the 2022–23 flu season. They found the mRNA version offered 34 per cent more efficacy. There was no placebo comparison.

"That actually is a significant increment of better protection in a single season with the strains that were circulating with this product," Saxinger said. "It's an important proof of concept that is worth pursuing."

In the study, mild to moderate side effects like fever, chills and redness were more common among those who received an mRNA influenza vaccine than the traditional jab.  

"Twenty-four hours of feeling lousy seems to be a feature of current mRNA [vaccines]," Miller said. "I think there are ways that the mRNA [vaccine technology] can evolve in the future to alleviate some of those systemic effects."