Vaxigrip Vaccine: How It Works, Side Effects, and Uses

Vaxigrip is one of the most widely used influenza vaccines in the world, manufactured by Sanofi Pasteur and available in more than 100 countries. The current version, marketed as Vaxigrip Tetra (or Vaxigrip QIV), is a quadrivalent inactivated split-virion vaccine that covers four influenza strains: two influenza A subtypes (H1N1 and H3N2) and two influenza B lineages (Victoria and Yamagata). Grown in embryonated chicken eggs using a manufacturing process that dates back decades, Vaxigrip occupies a particular niche in the flu vaccine landscape, with a strong safety record and real-world effectiveness data across age groups from infants to the elderly, though its performance varies from season to season and faces inherent limitations tied to how egg-based vaccines are made.

What Vaxigrip Tetra Actually Does in the Body

When you receive a Vaxigrip shot, your immune system encounters split fragments of the influenza virus, primarily the hemagglutinin surface protein. These fragments cannot cause influenza but are enough to trigger antibody production. In a phase 3 trial comparing Vaxigrip Tetra to another quadrivalent vaccine, immunization with Vaxigrip Tetra increased antibody levels roughly seven- to eightfold against A/H1N1, A/H3N2, and B/Victoria viruses, with about a threefold increase against B/Yamagata. The vaccine also induced high seroconversion rates, meaning a large proportion of recipients developed antibodies considered protective against those strains.¹

Beyond antibodies, influenza vaccination also engages cellular immunity. Specialized helper T cells activated by the vaccine support B cells in producing antibodies and forming immunological memory. Research has shown that influenza vaccination triggers coordinated responses from follicular helper T cells, which promote B-cell expansion and antibody secretion.² That said, newer vaccine platforms appear to generate stronger cellular responses. A study comparing a seasonal influenza mRNA vaccine with Vaxigrip found that both the mRNA vaccine and Fluad (an adjuvanted vaccine) produced higher frequencies of memory B cells and T cells specific to influenza compared to Vaxigrip, with the mRNA vaccine showing particularly strong responses in lymph nodes.³ This does not mean Vaxigrip fails to protect, but it suggests that its immune footprint is more modest than what some next-generation vaccines can achieve.

Real-World Effectiveness by Age Group

Vaccine effectiveness in the real world is always lower than what immunogenicity data from controlled trials would suggest, because real-world performance depends on how well the vaccine strains match what is circulating, the health of the person vaccinated, and the intensity of the flu season. During the 2018–2019 European flu season, the DRIVE network estimated brand-specific effectiveness for Vaxigrip Tetra at about 54% for children aged six months to six years and roughly 30% for adults aged 65 and older.⁴ Those numbers are typical of egg-based flu vaccines generally: moderate protection in younger people, weaker protection in older adults whose immune systems respond less vigorously.

The gap between age groups is consistent across flu seasons and reflects a well-understood immunological reality. Older adults produce fewer antibodies after vaccination and those antibodies wane faster. This is not unique to Vaxigrip; it is a feature of aging immune systems encountering any standard-dose inactivated flu vaccine.

Use in Children

Vaxigrip Tetra is approved for use from six months of age. Clinical trials evaluating the vaccine in children found safety and immunogenicity comparable to the older trivalent version (which covered only three strains instead of four). The quadrivalent formulation provided a stronger immune response to the additional B lineage strain without weakening the response to the other three. In children aged six to 35 months, preliminary data confirmed both safety and immunogenicity, with no concerning signals.⁵

Children under nine who are receiving an influenza vaccine for the first time typically need two doses spaced about four weeks apart to build adequate immunity. After that initial priming season, a single annual dose is standard. This two-dose schedule applies to all inactivated flu vaccines, not just Vaxigrip.

Vaccination During Pregnancy

Pregnant women are a priority group for influenza vaccination because the infection carries higher risks of complications during pregnancy, and newborns under six months are too young to be vaccinated themselves. A randomized trial comparing the quadrivalent Vaxigrip Tetra to the older trivalent version in pregnant women found similar safety and immunogenicity between the two formulations. Antibody levels in umbilical cord blood were roughly 1.5 to 1.9 times higher than in the mother’s blood at delivery, confirming that protective antibodies cross the placenta efficiently. No vaccine-related adverse events were reported in newborns.⁶

A real-world study of over 600 pregnant women during the 2019–2020 season found that vaccination with the quadrivalent vaccine reduced laboratory-confirmed influenza by about 44% in the mothers and roughly 31% in their infants during the first months of life.⁷ The infant protection comes entirely from passively transferred maternal antibodies, which gradually wane over the first few months. That window of passive protection is the only defense newborns have until they are old enough for their own vaccination at six months.

Side Effects and Safety Profile

Vaxigrip Tetra’s side effect profile is well characterized through both clinical trials and post-marketing surveillance. A large post-marketing study conducted in South Korea found that injection-site pain was the most common local reaction, reported by about 42% of participants. Muscle aches and general malaise were the most frequent systemic reactions, each reported by roughly 36–38% of recipients. The vast majority of these reactions were mild. Severe local or systemic reactions were rare, occurring in only about 1–2% of participants. No serious adverse events related to the vaccine were identified, and the most frequently reported unsolicited event was a common cold.⁸

These numbers are broadly consistent with what is seen across inactivated flu vaccines. Injection-site soreness and a day or two of fatigue or muscle aches are the norm, not the exception. A phase 4 study of Fluarix Tetra (a competing quadrivalent vaccine from GSK) in adults 65 and older found somewhat lower rates of injection-site pain at 15% and fever at about 7%, with no serious adverse events, which shows some variability between brands and populations but no concerning differences in the safety picture overall.⁹

The Egg Adaptation Problem

Vaxigrip, like most flu vaccines worldwide, is manufactured using chicken eggs. This process is proven and scalable but introduces a well-known complication: when influenza viruses are grown in eggs, they can mutate to better suit the egg environment, and those mutations sometimes change the very surface proteins the vaccine is supposed to teach your immune system to recognize. The result is a vaccine that trains your antibodies against a slightly different target than the virus actually circulating in the population.

This problem is not theoretical. During the 2012–2013 flu season, researchers found that the H3N2 component of egg-based vaccines had acquired mutations during manufacturing that made it antigenically distinct from the circulating virus, even though the circulating virus itself had not drifted away from the recommended vaccine strain. In other words, the poor match was the vaccine’s fault, not the virus’s.¹⁰ H3N2 is particularly susceptible to this issue, which partly explains why flu vaccine effectiveness tends to be lowest in years dominated by H3N2.

An expert consensus analysis estimated that egg adaptation reduces vaccine effectiveness across Europe by about 4–16%, depending on the strain, with the greatest impact on H3N2 in people under 65.¹¹ For context, natural antigenic drift, the normal season-to-season evolution of flu viruses, was estimated to cause a similar-sized reduction of 5–20%. So egg adaptation is roughly as damaging to effectiveness as the virus changing on its own, a sobering comparison that has pushed interest toward cell-cultured and recombinant vaccines that avoid eggs entirely.¹²

How Vaxigrip Compares to Enhanced and Next-Generation Vaccines

For older adults, several “enhanced” flu vaccines exist that aim to overcome the weakened immune response that comes with aging. High-dose vaccines contain four times the antigen of standard-dose formulations. Adjuvanted vaccines add an immune-boosting compound. Recombinant vaccines are made without eggs, sidestepping the adaptation problem entirely. Where does a standard-dose egg-based vaccine like Vaxigrip fit in this landscape?

A large trial published in the New England Journal of Medicine randomized over 300,000 adults aged 65 and older to high-dose or standard-dose flu vaccine and found that the high-dose version reduced influenza-related hospitalizations by about 44% relative to the standard dose. However, the difference in all-cause hospitalization was small and not statistically significant.¹³ A meta-analysis of multiple studies confirmed the pattern: high-dose vaccines reduced influenza hospitalizations by about 39% and lab-confirmed influenza by about 31% compared to standard dose, but the mortality difference was not significant.¹⁴

A network meta-analysis that tried to compare all enhanced vaccines head-to-head found that adjuvanted, high-dose, and recombinant vaccines each offered modestly increased protection compared to standard-dose vaccines like Vaxigrip, with relative effectiveness estimates ranging from about 10% to 19% in observational studies. But the differences between the enhanced vaccines themselves were small and not statistically significant.¹⁵ What this tells you practically is that for people 65 and older, enhanced vaccines are probably worth choosing over standard-dose Vaxigrip when they are available, but the advantage is meaningful mainly for influenza-specific outcomes, not for broader hospitalization or death from any cause.

For younger adults and children, where the immune system mounts a robust response to standard doses, the case for enhanced vaccines is weaker, and Vaxigrip’s effectiveness in the 40–60% range during well-matched seasons is considered adequate by most national immunization programs.

The Intradermal Route

For several years, Sanofi Pasteur marketed an intradermal version of its influenza vaccine, delivered via a tiny micro-needle into the skin rather than a standard intramuscular injection. The skin is densely packed with immune cells called dendritic cells, which makes it an attractive target for vaccination. In theory, you can use less antigen and still generate a strong response.

Clinical trials supported this idea, particularly in older adults. A three-year randomized trial in over 3,700 adults aged 60 and older found that an intradermal vaccine containing the same 15 micrograms of antigen per strain as the standard intramuscular vaccine produced statistically superior antibody levels and seroprotection rates.¹⁶ A separate trial in over 1,100 older volunteers confirmed this advantage: for every strain, the intradermal vaccine produced superior antibody titers compared to the intramuscular control.¹⁷

Interestingly, a dose-finding study showed that lower intradermal doses of 3 or 6 micrograms were less immunogenic than the full 15-microgram intramuscular dose, while 9 micrograms delivered intradermally matched the standard intramuscular dose across all strains.¹⁸ This suggested a dose-sparing potential of about 40%, which could matter during pandemic manufacturing when antigen supply is limited. However, the intradermal product (marketed as Intanza or IDflu) was eventually withdrawn from several markets for commercial reasons, not safety concerns. The micro-needle delivery system was more expensive to produce and the logistics were more complex, which limited uptake despite the immunological advantages.

Co-administration With Other Vaccines

A question that comes up frequently is whether Vaxigrip can be given at the same time as other vaccines. In general, inactivated flu vaccines can be administered simultaneously with most other routine vaccines at separate injection sites. One area of particular research interest has been co-administration of influenza and pneumococcal vaccines, since the populations at highest risk for both infections overlap considerably.

A study of 100 patients with type 2 diabetes who received both Vaxigrip and Prevnar 13 (a pneumococcal vaccine) found significant improvements in respiratory symptoms and inflammatory markers after vaccination. During the COVID-19 pandemic, only 4% of the vaccinated group developed mild COVID-19 with no complications, compared to about 66% of unvaccinated diabetic controls who developed severe COVID-19. The hazard ratio for death from COVID-19 was over 10 times higher in the unvaccinated group.¹⁹ This was a small, non-randomized study, so the COVID-19 findings should be interpreted cautiously. But it does suggest that co-administration of influenza and pneumococcal vaccines is safe and may have broader respiratory benefits in high-risk populations.

Immune Response in People With Chronic Conditions

How well flu vaccines work in people with chronic health conditions is an underappreciated question. The immune response to vaccination can differ meaningfully depending on the underlying condition. A study of patients with chronic fatigue syndrome who received Vaxigrip found that vaccination induced a significant increase in cellular immune proliferation in patients that was actually not observed in healthy controls. Both groups produced functional T-cell responses, but the pattern of cellular activation differed.²⁰ This finding does not mean the vaccine works better in people with chronic fatigue syndrome. Rather, it illustrates that the immune backdrop against which a vaccine operates can shape the response in unexpected ways.

For immunocompromised individuals, such as those receiving chemotherapy, organ transplant recipients on immunosuppressive drugs, or people with HIV, inactivated vaccines like Vaxigrip are generally considered safe because they contain no live virus. However, the antibody response may be blunted, sometimes substantially. Many guidelines recommend vaccinating these patients anyway, reasoning that partial protection is better than none, and that herd immunity from vaccinated household contacts provides an additional layer of defense.

Why Effectiveness Varies So Much From Year to Year

If you follow news coverage of flu vaccine effectiveness, you have probably noticed wild swings from one season to the next. One year effectiveness might be estimated around 50–60%, and the next it drops to 10–20%. Three factors drive most of this variation, and understanding them helps explain both the value and the frustration of annual flu vaccination.

The first is strain match. The WHO selects vaccine strains months before flu season begins, based on surveillance data about which viruses are circulating. If the virus drifts between the selection date and the season’s peak, the vaccine may target a strain that no longer closely matches what is making people sick. The second, as discussed earlier, is egg adaptation, which can degrade the match even when the WHO picks the right strain. The third is the dominance of particular subtypes. Seasons dominated by H3N2 consistently show lower vaccine effectiveness than seasons dominated by H1N1 or influenza B, in part because H3N2 mutates faster and is more affected by egg adaptation.

None of these factors is unique to Vaxigrip. Every egg-based flu vaccine faces the same challenges. Cell-cultured and recombinant vaccines avoid the egg-adaptation piece, but they are still subject to strain mismatch from antigenic drift. The annual reformulation cycle remains the fundamental constraint on how well any flu vaccine can perform in a given year.

Storage, Handling, and Practical Considerations

Vaxigrip Tetra is supplied as a pre-filled syringe and must be stored at 2–8°C (standard refrigerator temperature). It should not be frozen, as freezing damages the antigen and renders the vaccine ineffective. Once removed from refrigeration, it should be used promptly. The vaccine is a clear, colorless liquid, and any discoloration or particulate matter is a reason to discard the dose.

For adults and children over 36 months, the dose is 0.5 mL given intramuscularly, typically in the deltoid muscle of the upper arm. For children 6–35 months, a 0.25 mL dose is used in many countries, though some national guidelines now recommend the full 0.5 mL dose for this age group as well, reflecting updated evidence that the higher dose is safe and may improve immunogenicity without increasing adverse events. Your country’s specific labeling will dictate which dose applies.

People with a severe allergy to eggs are sometimes told they cannot receive egg-based flu vaccines. Current guidelines in many countries have relaxed this restriction substantially. Most people with egg allergy, even those who experience hives after eating eggs, can safely receive standard inactivated flu vaccines in a healthcare setting prepared to manage allergic reactions. The amount of egg protein in modern split-virion vaccines like Vaxigrip is extremely low. Only individuals with a history of severe anaphylaxis to eggs may need to consider egg-free alternatives like recombinant vaccines, and even then, many allergists consider standard vaccines safe with appropriate observation.