Egg-based, strain-matched development is the primary reason flu-vaccine efficacy is difficult to improve

What's this about?

People disagree about whether making flu shots in eggs is the main reason doctors struggle to improve them.

Eggs can cause problems, but they do not explain everything.

What supporters say

• Egg-grown flu viruses can change while they grow, which can make the shot match real flu less well.
• This problem often affects H3N2, a flu type that can cause harsh flu seasons.
• In some H3N2 seasons, egg-based shots gave weak protection, which fits this concern.
• Shots grown in cells or made without eggs sometimes protect people better than egg-based shots.

What critics say

• Flu viruses can change fast in people, even when makers avoid eggs.
• Shot makers must pick flu types months before the flu season starts.
• That long wait can lead to a poor match between the shot and the flu that spreads.
• Studies often find cell-grown shots work about as well as egg-based shots, not always better.

The bottom line

Egg growth can make flu shots work less well, mainly for some H3N2 viruses.

But it is one big problem, not the main reason flu shots remain hard to improve.

Flu vaccines made in eggs can sometimes be less effective because the virus changes during production. But the evidence shows that egg-based manufacturing is one important problem, not the main reason overall that flu-vaccine effectiveness remains hard to improve.

The case for

The strongest case against egg-based production concerns the H3N2 strain, which has often caused severe flu seasons. Growing vaccine viruses in eggs can force them to adapt in ways that alter the surface features targeted by the immune system. As a result, antibodies produced after vaccination may recognize the egg-grown virus better than the virus actually circulating in people. These egg-adaptive changes can create a meaningful mismatch between the vaccine and the season’s flu viruses 1.

Laboratory research on recent H3N2 viruses has identified a change involving a sugar coating on the virus that affected how antibodies reacted to egg-adapted vaccine strains. Reviews by experts have concluded that such changes can reduce vaccine effectiveness, especially during some H3N2 seasons. In 2017–18, for example, U.S. estimates found particularly weak protection against H3N2, a pattern consistent with this concern (see Figure 1).

There is also evidence that alternatives can avoid some of these changes. Cell-based and recombinant vaccines do not rely on egg propagation, and studies have sometimes found that they provide better protection than standard egg-based products. A review and meta-analysis found cell-based vaccines were generally similar to egg-based vaccines but sometimes performed better, while studies of recombinant vaccines suggest they may improve protection in particular groups or circumstances 2.

That makes manufacturing technology a potentially changeable part of the problem. Moving away from eggs may be especially useful when the circulating virus is H3N2 and egg adaptation has caused a major antigenic change.

The case against

Still, flu viruses can change after health authorities have selected the strains for a vaccine. This process, known as antigenic drift, can leave the vaccine poorly matched to circulating viruses regardless of whether it was made in eggs, cells or another system. A U.S. analysis across multiple seasons linked lower vaccine effectiveness to the antigenic distance between vaccine strains and the viruses that ultimately spread (see Figure 2). Viral evolution is therefore an independent limit on protection 3.

H3N2 is particularly difficult for reasons beyond egg adaptation. It evolves quickly, and people arrive at vaccination with very different immune histories from earlier infections and vaccinations. Those histories can shape their response in ways that manufacturing changes alone cannot solve.

Age and the timing of vaccination also matter. Older adults often produce weaker or less consistent immune responses because the immune system becomes less responsive with age. A review of randomized trials found reduced or variable vaccine immune responses in elderly people, a limitation that would remain even if egg production disappeared 4. Protection can also fade during the course of a flu season.

Most importantly, comparisons between vaccine platforms have not shown a consistent, large advantage for non-egg products. A multi-season observational study found only modest differences between cell-based and egg-based vaccines, and those differences varied by strain and season. A randomized antibody study found limited differences in immune responses but did not establish better real-world clinical protection. Other analyses likewise found that product performance varied across comparisons, while CDC estimates still differ by age, care setting and virus subtype even as non-egg options are available 5.

The evidence has limits. Many platform comparisons are observational or examine specific products, and stronger antibody responses do not automatically mean better protection from illness. Some studies may also involve manufacturer funding or affiliations, making conflicts of interest a continuing concern.

The bottom line

Egg adaptation is a real and sometimes substantial cause of weaker flu-vaccine protection, particularly in H3N2 seasons. But the evidence does not support the broader claim that egg-based, strain-matched development is the primary reason flu-vaccine efficacy has been difficult to improve.

Instead, the problem has several major causes: viruses drift after strains are chosen, immune responses vary widely—especially in older adults—and protection may wane over time. Cell-based, recombinant, higher-dose or adjuvanted vaccines may help selected groups and seasons, but their benefit depends on the strain, the product and the people receiving it.

Confidence in this conclusion is high. Different kinds of evidence agree that egg-based changes can cause harm, while also showing that viral evolution and human immune biology independently limit how well seasonal flu vaccines can work.