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

What's this about?

People disagree about whether egg-grown flu shots are the main reason flu shots can be hard to improve. Egg growth matters, but other causes also affect how well shots work.

What supporters say

• Flu germs can change while growing in eggs, making them less like the flu germs spreading among people.
• Cell-grown and other non-egg shots can give better protection in some flu seasons.

What critics say

• People's past flu shots, past flu sickness, age, and health can change how well a shot works.
• Flu germs keep changing after health experts pick the flu types for the shot.
• Shot protection can grow weaker as the flu season goes on.

How to read this

The number of points on each side does not show which side is right; strong proof matters more than a long list.

The bottom line

Egg growth can hurt the match between a shot and the flu, especially in some H3N2 seasons. However, the evidence does not show that egg growth alone causes the main problem in every season.

The claim that egg-based, strain-matched production is the main reason flu-vaccine effectiveness is hard to improve has some important support. But the wider evidence points to several causes, rather than one dominant explanation in every season.

The case for

Egg-based production can change the virus used to make a vaccine. As the virus adapts to growing in eggs, it may become less similar to the influenza viruses spreading among people. That can leave the vaccine targeting a poorer match, particularly in some H3N2 seasons. A systematic review and meta-analysis found that these egg-related mutations can reduce vaccine effectiveness, while comparisons of egg- and cell-based virus samples identified mutations that may contribute to mismatch. This is a direct link between a manufacturing step and weaker protection 1. The meta-analytic evidence is summarized in the comparative evidence shown in Figure 2.

Alternative technologies—including cell-based, recombinant and other non-egg vaccines—can avoid some of these changes. Studies have found differences in immune responses between platforms, depending on the strain and season. A randomized pragmatic trial also found measurable differences between platforms. These findings suggest that moving away from eggs can sometimes improve protection, although they do not show that it solves the wider problem on its own 2.

The argument is therefore strongest in a limited sense. Egg adaptation is a real and actionable barrier, especially when a particular strain and season are affected. But the evidence is less clear that it is the main cause of difficult-to-improve effectiveness across all influenza vaccines and seasons.

The case against

Influenza viruses continue to evolve after vaccine recommendations are made. Global health authorities must choose vaccine strains months before widespread transmission, so the virus may drift away from the predicted target even when egg-related changes have been avoided. Antigenic drift is a separate source of mismatch, not simply another form of the egg-production problem 3.

The person receiving the vaccine also matters. Older age, underlying illness, previous exposure to influenza and immune “imprinting” can all change how strongly someone responds. These factors can reduce or vary protection whether the vaccine was made in eggs, cells or through recombinant methods 4.

Protection may also weaken as the season progresses. This means vaccination timing can affect measured effectiveness, even when the vaccine is well matched and made without eggs 5. Effectiveness varies across seasons, influenza subtypes, age groups, medical settings and outcomes. The relationship between effectiveness and antigenic distance across multiple seasons is shown in Figure 3, while Figure 4 illustrates differences by age, subtype and clinical outcome.

These findings point to a broader challenge. Improving vaccines may require not only different production methods, but also wider antigen coverage, better immune targeting and improved formulations. Differences in antibody responses across egg-based, cell-based and recombinant vaccines, early work on mosaic haemagglutinin designs, and a phase 2 study of an adjuvanted cell-derived vaccine all suggest possible routes beyond replacing eggs.

The bottom line

Egg adaptation is an important contributor, but the evidence does not show that it is the primary reason overall that flu-vaccine effectiveness has been difficult to improve. Confidence is high that egg-related changes can materially weaken antigenic similarity in particular settings, especially some H3N2 seasons. Confidence is lower that this effect ranks above viral evolution, host immunity, waning protection and vaccine design across the full range of seasons and populations.

The evidence therefore favours the case against the claim, but not overwhelmingly. Alternative platforms are a meaningful part of the solution, rather than a standalone fix. The main unresolved question is comparative: how much improvement would come from removing egg adaptation, compared with addressing viral drift, differences in immune response, waning protection, antigen breadth and formulation?