What Goes Into Making Our Seasonal Flu Jab? – Expert Q+A

Friday, 2 May 2025, 10:51 am
Press Release: Science Media Centre

How are the eggs to make traditional seasonal flu vaccines procured and kept safe from other pathogens, like avian influenza? Why do seasonal flu vaccines work better some years than others? And what other kinds of flu jabs are there?

Most seasonal influenza vaccines are made using chicken eggs.

The SMC asked experts for background on seasonal flu vaccine manufacture and alternatives to the standard jab.

Dr Mary Nowlan, Senior Advisor, Immunisation Advisory Centre (IMAC), comments:

Where do the eggs come from to make the seasonal influenza vaccine?

“Egg-based influenza vaccine manufacture is a well-established process, with chicken eggs being the preferred medium to grow viruses since the 1950s.

“Large quantities of eggs are produced in specific pathogen-free chicken colonies. These are controlled environments in which the hens and the eggs are closely monitored, which means that every step is taken to ensure that these animals are not at risk of being infected by bird influenza strains (including H5N1).”

Why do influenza vaccines work better some years than others?

“Predicting and matching exact influenza strains is an imperfect science, and vaccines that are developed are not always a perfect match for the circulating virus.

“Growing enough virus for millions of doses of vaccine is a slow process, particularly in eggs. As the virus grows in eggs, small changes occur in the virus – known as ‘egg-adaption’. This can mean that the virus produced does not completely match the circulating virus, thereby potentially reducing the effectiveness of the vaccine.

“Another reason that the vaccine effectiveness can alter each year is that the wild virus strains have also mutated over the six months since they were selected for the vaccine and a mismatch can also occur. Advances in technology to analyse the circulating strains may have mitigated that risk to some degree, but the unpredictability of influenza virus evolution means mismatches still occur.”

How is it decided which strains are covered by the flu vaccines for the upcoming flu season?

“As the influenza virus continually mutates, vaccine production must take this into account and adjust the virus strains for the following influenza season.

“The Global Influenza Surveillance and Response System collates data from the WHO Collaborating Centres, who in turn receive data from the National Influenza Centres (ESR in New Zealand).

“This provides data for both the Southern and the Northern Hemispheres to help to predict which influenza strains are likely to circulate during their respective flu seasons each year.

“At around six months before the next influenza season, the WHO holds a meeting (in October and April – one for each hemisphere) to decide on the composition of upcoming influenza virus vaccines. Once this has happened, production of the next year’s vaccine begins.

“The reason that influenza vaccine is recommended annually is due the constant changes in the influenza virus strains, and therefore the vaccine needs to alter in response. Protection only lasts for a short time.”

What are the alternatives to the standard flu vaccine?

“All the current seasonal influenza vaccines rely on recommendation from the WHO as to which influenza strains are included. Vaccines available in New Zealand include a cell-based vaccine, in which the virus is grown in cell culture rather than eggs, and an ‘adjuvanted’ vaccine, in which a naturally occurring compound (squalene) is added to induce a stronger immune response in older people who may have a weaker immune response than young adults.

“Further vaccines available internationally but not New Zealand include recombinant influenza vaccines, in which the surface proteins of the virus are produced artificially rather than being isolated from whole viruses. Also available in the Northern Hemisphere is a live attenuated influenza vaccine that is given through a nasal spray, usually to children.”

Why should we be concerned about bird flu in New Zealand?

“Birds, particularly wading birds, carry influenza virus in their guts. Usually this does not make the birds sick. However, some forms of avian flu have become deadly. Highly pathogenic avian influenza (HPAI) is of significant concern, not only for the risk of spread to humans but for wild and domestic birds and other animals such as livestock. H5N1 is just one strain of avian influenza that can mutate to make animals sick. It has also infected humans who were exposed to sick animals.

“As well as concerns about infection in birds, livestock, and marine mammals in New Zealand, a big concern in humans would be if a HPAI infected someone who is also infected with the seasonal influenza virus. The risk is that the influenza viruses would combine into a strain that is transmissible between humans. This has the potential to cause a pandemic. People who work with animals are encouraged to receive the seasonal influenza vaccine each year to reduce this risk.”

No conflicts declared.

Dr Lisa Connor, Programme Leader, Infection and Vaccinology Group, Malaghan Institute of Medical Research, comments:

Could the ongoing impact of the H5N1 avian flu pandemic on raised chickens overseas have flow-on effects on influenza vaccine availability in NZ?

“One widely used influenza vaccine in New Zealand is the Tetra Fluvac, which provides protection against four different flu virus variants. This vaccine is produced using eggs and is available for free to eligible New Zealanders, making it an accessible and effective option.

“However, it is important to note that there are also other flu vaccines available on the market that do not rely on egg-based production. These vaccines are made using cell cultures and, while not currently funded by the NZ government, they are an alternative for those who require or prefer them. These non-egg-based vaccines are available in New Zealand but are typically more expensive, as they are not covered by the public funding system.

“H5N1, the avian influenza strain, is a significant concern, especially for our bird population, including chickens. Thus, there is the potential to disrupt egg production and, consequently, vaccine production. Fortunately, the availability of cell culture-based vaccines ensures that there are safe, effective, and approved alternatives on the market that do not depend on eggs. This flexibility is reassuring, as it means that if egg production were to be impacted by H5N1 or other factors, we have viable options to ensure continued vaccine supply.

“Overall, the current flu vaccines, including the egg-based Tetra Fluvac can lower the risk of infection and reduce severity of disease from influenza, and many New Zealanders are eligible for free vaccination. Should the situation evolve and potential shortages arise, the availability of mammalian cell culture-based vaccines offers a solid backup. We are fortunate that there are multiple, effective vaccine options in New Zealand, ensuring that the public remains well-prepared to limit infection from influenza, regardless of future challenges.”

No conflicts of interest.

Natalie Netzler PhD, Senior Lecturer, Faculty of Medical and Health Sciences, University of Auckland, comments:

“The current egg-based vaccines are produced in a manufacturing system that is over 70 years old. However, there is a lengthy lead time needed to secure the eggs required to make the egg-based flu vaccine.

“Following the 2009 influenza pandemic we had a shortage of egg-based vaccines due to a number of issues including slow virus growth of the pandemic strain in eggs and a very high demand coupled with the slow manufacturing process.

“Given that our Indigenous populations all over the world face higher rates of severe influenza compared to non-indigenous groups in the same regions, it is important that we have sufficient vaccine supplies to protect our unique Māori and Pacific communities here in Aotearoa NZ.

“The development of cell-based influenza vaccines is gaining traction. While there are some challenges posed by these newer flu vaccines including higher costs of production, and limited global availability, there are several advantages of cell-based vaccines over egg-based flu shots. These include faster and more predictable rates of production and being able to offer an egg-free option.

“Although our current influenza vaccines are not perfect in that they don’t always stop you getting the flu altogether, they do offer protection against severe flu and are highly recommended for those at risk of severe disease, no matter which type of flu vaccine you get.”

Conflict of interest statement: “I work with several Pacific and Māori organisations and health providers to support our communities to make informed decisions on immunisation.”

Sue Huang, Director, WHO National Influenza Centre, Institute of Environmental Science and Research (ESR), comments:

What strains are covered in this year’s flu vaccines?

“The southern hemisphere influenza vaccines to be used in NZ in 2025:

Egg-based vaccines:

• an A/Victoria/4897/2022 (H1N1)pdm09-like virus;
• an A/Croatia/10136RV/2023 (H3N2)-like virus; and
• a B/Austria/1359417/2021 (B/Victoria lineage)-like virus.

Cell culture-, recombinant protein- or nucleic acid-based vaccines

• an A/Wisconsin/67/2022 (H1N1)pdm09-like virus;
• an A/District of Columbia/27/2023 (H3N2)-like virus; and
• a B/Austria/1359417/2021 (B/Victoria lineage)-like virus.

The recommendation for the B/Yamagata lineage component of quadrivalent influenza vaccines remains unchanged from previous recommendations:
• a B/Phuket/3073/2013 (B/Yamagata lineage)-like virus.

“Interestingly, the influenza vaccine strains recommended by WHO in February 2025 for the use for northern hemisphere countries in 2025-2026 are the same as the NZ’s vaccine strains to be used in 2025. This suggests that our vaccine strains match well with the current circulating viruses which would give us optimal protection. Influenza vaccination is the primary tool to protection us against influenza, particularly for those vulnerable groups (elderly, and individuals with underlying conditions).

“At the moment, influenza activity is still at a low level. Influenza A(H1N1)pdm09 is the predominant strain followed by influenza B and A(H3N2). For details, see here.”

No conflict of interest.

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