Flu vs COVID-19 vaccines how they compare
, by Andrew Odgers, 8 min reading time
Vaccine Comparison
Flu vs COVID-19 Vaccines: How They Compare
Flu and COVID-19 vaccines both protect against respiratory viruses that cause serious illness and death each winter. They work differently, are updated on different schedules, and cannot replace each other. This guide sets out the key similarities and differences clearly, including eligibility, technology, effectiveness, and what to expect if you have both at once.
AnnualFlu vaccine reformulated every year
Same apptBoth can be given together
NeitherReplaces the other
40,000+Typical UK flu deaths in a severe season
Side-by-side comparison
Flu and COVID-19 vaccines at a glance
| Feature | Seasonal Flu Vaccine | COVID-19 Booster (UK) |
|---|---|---|
| Target | Influenza A and B (4 strains each year) | SARS-CoV-2 (updated for circulating variants) |
| Technology (main UK types) | Inactivated quadrivalent (most adults); live-attenuated nasal spray (children) | mRNA (Pfizer/Moderna) or protein subunit (Novavax) |
| Annual reformulation | Yes — WHO selects strains each February and September | Yes — updated to match circulating variants |
| Number of doses | One dose per season | Primary course plus annual booster for eligible groups |
| Route | IM injection (adults); nasal spray (children 2–17) | IM injection |
| NHS eligibility | 65+, at-risk groups, children 2–16, pregnant women, carers, healthcare workers | 65+, immunosuppressed, care home residents, some 50–64 |
| Can they be given together? | Yes — co-administration is approved | Yes — same appointment, different arms is recommended |
| How soon protection develops | ~2 weeks after vaccination | 2–4 weeks after final dose |
| Duration of protection | ~3–6 months (wanes and strains drift) | Several months, declining over time |
Table based on current UK NHS programme guidance. Check NHS.uk for the most current eligible groups.
Why they cannot replace each other
Two completely different viruses require two different vaccines
Influenza and SARS-CoV-2 are unrelated viruses
Influenza A and B viruses belong to the Orthomyxoviridae family. SARS-CoV-2 is a betacoronavirus. They share no meaningful surface proteins. Antibodies produced against influenza haemagglutinin or neuraminidase do not recognise the SARS-CoV-2 spike protein. There is no cross-protection between the two vaccines, and receiving one provides zero protection against the other disease.
They peak in the same season
Both flu and COVID-19 circulate predominantly in autumn and winter in the UK. Co-infection — being infected with both viruses simultaneously — has been documented and produces more severe disease than either alone. Protecting against both is therefore especially valuable for high-risk groups during the winter period.
Co-administration is safe and practical
Multiple studies including the UK ComFluCOV trial confirmed that giving both vaccines at the same appointment in different arms produces equivalent immune responses to giving them at separate appointments. Side effects are not meaningfully increased by co-administration. The NHS offers both vaccines together to eligible individuals to reduce appointment burden.
How flu vaccine effectiveness is measured
Why flu effectiveness varies between years
| Season | Dominant Strain | Estimated Effectiveness (UK) | Notes |
|---|---|---|---|
| 2019–20 | A(H1N1)pdm09 | 40–50% | Good match season |
| 2020–21 | Minimal circulation | High (by proxy) | COVID restrictions suppressed flu |
| 2021–22 | A(H3N2) | 25–35% | H3N2 seasons typically harder to match |
| 2022–23 | Mixed A and B | 45–55% | Return of normal flu season post-pandemic |
| 2023–24 | A(H1N1) dominant | 50–60% | Good strain match |
| 2024–25 | A(H3N2) resurgent | 30–45% (estimate) | H3N2 drift challenge |
Effectiveness estimates from PHE/UKHSA seasonal reports. COVID-19 booster effectiveness follows a similar pattern of initial high efficacy followed by waning.
UK COVID-19 booster effectiveness against hospitalisation over time (approximate)
Side effects: flu vs COVID-19 booster
TrueWhat to expect from each
Frequently asked
Questions answered
Does having flu this year mean I don't need the flu vaccine next year?
Natural flu infection produces immunity to the specific strain you were infected with. However, the following year's vaccine is formulated against a new set of predicted strains. Past natural infection does not protect against next season's dominant strains, and vaccination is still recommended each year regardless of prior infection.
Why is flu vaccine effectiveness lower than some other vaccines?
Influenza vaccines face two challenges that most other vaccines do not: rapid antigenic drift means the virus may have changed by the time the vaccine is given, and the strain selection for each year's vaccine is made 6 to 9 months in advance based on surveillance predictions. In years where the prediction closely matches the circulating strains, effectiveness is higher; in H3N2 dominant years with significant drift, effectiveness is lower.
Can I have the flu jab and COVID booster in the same arm?
No. When both vaccines are given at the same appointment they should be administered in different arms. This prevents local reactions from both vaccines occurring at the same site, which would make it harder to assess which caused any particular local reaction.
I had COVID last month — should I still have the booster?
Current UK guidance recommends waiting at least 4 weeks after confirmed COVID-19 infection before receiving a COVID-19 booster. Natural infection provides a temporary boost to immunity; waiting allows that initial post-infection immunity to begin to wane before the booster is given, which may improve the overall response.
Vaccination supplies
Clinical vaccination consumables from Charles Medical
Charles Medical supplies hypodermic needles, syringes, and all consumables used in vaccination practice. Next-day UK delivery, no minimum order.