Printed on: November 21st, 2025

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Seasonal influenza

Seasonal influenza

Influenza is a highly-infectious acute upper respiratory tract infection caused by the viruses influenza A, B and C.

In the UK, most cases occur during an 8–10-week winter period. Influenza A is responsible for most epidemics, and causes a more severe clinical infection. Influenza B tends to cause smaller outbreaks with less severe disease, although children can be more affected.

Influenza activity in the UK is monitored by the UKHSA, which provides regular updates on the levels of infection, and guidance on immunisation and the use of antiviral treatments in primary care. We strongly recommend that you check the UKHSA for the most recent guidance because this changes frequently (gov.uk - seasonal influenza: guidance, data and analysis).

In this article, we draw on the UKHSA guidance on when to use antivirals, The Green Book, NICE guidance on antivirals for the treatment of influenza, and some older systematic reviews into the efficacy of antiviral treatments (Green Book Chapter 19 (updated 2025); NICE 2008, TA158; NICE 2009 TA168, BMJ 2014;348:g2545, Lancet 2014;2:395).

This article was updated in November 2025.

The Green Book reminds us of the characteristic features of influenza infection (Green Book Chapter 19 (updated 2025)):

• Incubation 1–3d.
• Presenting symptoms: fever, chills, headache, myalgia and fatigue, commonly seen alongside a dry cough, sore throat and blocked nose.
• Transmission: spread by droplet and aerosol, or by direct contact with the respiratory secretions of an infected person.
• Duration: for most healthy people, the infection will be unpleasant but self-limiting, lasting 2–7 days.

Certain groups are at higher risk of severe disease and complications, including children <6m; those with underlying respiratory or cardiac disease, chronic neurological conditions, BMI ≥40 or immunosuppression; and pregnant women.

Complications include:

• Bronchitis.
• Secondary bacterial pneumonia.
• Otitis media (in children).
• Less commonly: meningitis, encephalitis.

Most influenza vaccines are inactivated, but the nasal preparation is a LIVE attenuated vaccine (so should not be given to those who are immunocompromised).

The nasal preparation contains porcine gelatine. For those unwilling to have a porcine gelatine-containing vaccine for any reason, we can offer alternatives (UKHSA Guidance accessed November 2025, gov.uk – vaccines and porcine gelatine).

The WHO monitors the epidemiology of circulating influenza strains, and makes annual recommendations as to which strains should be included in the vaccines for the following year.

The JCVI updates the recommendations on which groups should be included in the immunisation programme and which vaccines should be used for each group. We recommend that you check the advice for your country:

• England: gov.uk - annual flu programme.
• Northern Ireland: hscni - influenza programme.
• Scotland: NHS Scotland: Winter programme 2025 – seasonal flu and COVID-19 vaccination.
• Wales: The national influenza immunisation programme 2025 to 2026 (WHC/2025/020).

There is a risk that antiviral medications used to treat flu may lower the effectiveness of the live attenuated (nasal) influenza vaccine. We should delay live vaccination until 48h after stopping anti-influenza antivirals. If a person requires anti-influenza antivirals within 2 weeks of live vaccination, the effectiveness of the vaccine may be reduced (Green Book Chapter 19 (updated 2025)).

The UKHSA guidance asks us to assess the severity of the influenza illness for our patient, alongside their risk factors for severe disease, to determine the treatment needed.

These groups are at risk:

• Chronic neurological, hepatic, renal, respiratory or cardiovascular disease.
• Diabetes mellitus (type not specified; we presume all).
• Immune suppression.
• BMI ≥40.
• Age <6m or >65y.
• Pregnancy (plus 2w postpartum).

The UKHSA published guidance on influenza treatment in November 2025.

This update does not mention waiting for the Chief Medical Officer to signal that antiviral treatment should be offered from primary care. Instead, the guidance suggests that antiviral treatment should be considered when the national influenza surveillance report indicates that flu is circulating above baseline levels or where there is increased circulation locally.

Who needs treatment? In summary:

• People with severe influenza (who will likely be admitted to hospital).
• People with non-severe influenza but in an ‘at-risk’ group listed above.
• When an independent prescriber believes the patient is at risk of developing serious complications from influenza, despite not being in the ‘at-risk’ list.

The UKHSA also states that treatment may be considered to reduce transmission to severely immunocompromised household contacts. This is a severe subset of those with broader ‘immune suppression’. See the UKHSA guidance for the full list.

In the community, oseltamivir is first line (licensed from age 12m; thought safe in pregnancy) and zanamivir is second line (licensed from age 5y; caution in pregnancy). Check the BNF for dose, duration and other details, e.g. if using in renal impairment, longer course duration in immune compromise.

NICE states that antivirals should be used if the person can start treatment within 48h (oseltamivir) or 36h of onset (zanamivir) (NICE 2009, TA168). The SPC states that “efficacy has been demonstrated when treatment is initiated within 2 days”. However, the UKHSA guideline does not make reference to a timeframe when starting antivirals to treat influenza. It states that the drugs perform best early in the illness, but doesn’t rule out their use (off licence) beyond 48h for more severe cases.

If ALL the following criteria are met, post-exposure prophylaxis may be issued (NICE 2008, TA158; NICE 2009, TA168):

• There has been contact with a person known to have (or strongly suspected to have) influenza.
• The person is in an ‘at-risk’ group.
• The person has not been vaccinated OR vaccination was given within 14 days of contact.
• Prophylaxis can be started within 48h (oseltamivir) or 36h (zanamivir).

Notes:

• In a localised outbreak such as a residential home or other institution, post-exposure prophylaxis can be considered regardless of vaccination status. Seek specialist health protection team advice about local incidents.
  • For outbreaks in care homes, we should follow the UKHSA guidance on managing outbreaks in care homes (July 2024, accessed November 2025), which defines how many cases, over what time period, constitute ‘an outbreak’.
  • The health protection team should be informed of a suspected outbreak in a care home (this can be done by the care home).
• Neonatal exposure: see the source UKHSA document (or speak to your local paediatric department).

Prophylaxis is given as oseltamivir or zanamivir once daily for 10 days.

For oseltamivir-resistant influenza, zanamivir is first line and baloxavir marboxil second line (UKHSA, November 2025).

Baloxavir marboxil reduces replication of influenza A and B viruses (BNF, accessed November 2025). It is given as a single oral dose. Avoid if pregnant or breastfeeding. It is expensive (£100 per tablet at the time of writing). The NICE technology appraisal was terminated because the manufacturer did not provide an evidence submission (NICE 2021, TA732).

The UKHSA guidance discusses testing with lateral flow antigen tests (which have good specificity but lack sensitivity) or the more sensitive reverse transcriptase polymerase chain reaction tests. Remember that prevalence affects the positive predictive values of these tests. Here are the key points:

• All admitted with severe influenza should be tested.
• Cases of confirmed influenza who are in contact with an immune-compromised or critically-ill person should undergo diagnostic sampling to check for antiviral resistance (but this should not delay prophylaxis for those contacts).
• ‘Out of season’ (i.e. during times when circulating virus is thought low): treatment may be guided by influenza tests.
• Otherwise, testing does not dictate decision to treat and doesn’t fall into the diagnostic guidance discussed above.
• ‘In season’: undertake testing on the index case IF AVAILABLE.
• If testing suggests someone on a prophylactic antiviral dose has influenza, a treatment course should replace the prophylactic course.

A systematic review and network meta-analysis in the Lancet looked at the evidence for post-exposure prophylaxis to prevent symptomatic influenza infection using 6 different antiviral drugs: zanamivir, oseltamivir, laninamivir, baloxavir, amantadine and rimantadine (Lancet 2024;404:764).

baloxavir, amantadine and rimantadine (Lancet 2024;404:764).

• The review found that prophylaxis with zanamivir, oseltamivir, laninamivir and baloxavir “probably” (moderate-certainty evidence) achieves clinically significant reductions in symptomatic influenza infections in populations at high risk of severe disease, but has little-to-no effect in lower-risk groups.
• There was also low-certainty evidence that these drugs may reduce risk of zoonotic infection with novel influenza viruses.
• Amantadine and rimantidine did not show significant benefit, even in high-risk populations, when used as post-exposure prophylaxis against influenza A or novel zoonotic influenza viruses.

A systematic review of neuraminidase inhibitors found that these antiviral medications were effective at preventing symptomatic influenza, reducing cases by 55%, with a NNT 33, but there was no evidence of reduction in transmission or reduction in admissions to hospital (BMJ 2014;348:92545).

For those with severe influenza, antivirals have clinical benefit such as reducing hospital stay and mortality (UKHSA, November 2025), but the evidence supporting widespread use in prevention of complications is sparse. In trials on adults, oseltamivir reduced the duration of symptoms by 0.5–1.5 days if given within 48h of onset of symptoms for laboratory-confirmed flu. This is not a clinically significant improvement in healthy adults (BMJ 2014;348:92545).

You only need to treat 28 people to cause 1 side-effect of nausea. Treatment also increases the risk of headaches (NNH 32) and psychiatric syndromes (NNH 94) (BMJ 2014;348:92545).

So, the answer is: antivirals do work, but only show modest benefits in the healthy population, and carry risks of nausea, headache, renal impairment and psychiatric symptoms. In higher-risk groups, the benefits are greater. We can use this to explain to our fit population suffering flu symptoms why we are saving antiviral agents for their frail neighbour.

Seasonal influenza Influenza is a highly-infectious acute upper respiratory tract infection caused by the influenza A, B and C viruses. Treatment is generally limited to those with severe influenza (likely in hospital) or those with non-severe influenza who belong to an ‘at-risk’ group. Those with more severe influenza gain most benefit from treatment (in terms of mortality and length of hospital stay). Benefits for treating ‘low-risk’ groups are marginal. Prophylaxis is considered for unvaccinated (or vaccinated <14d) contacts in ‘at-risk’ groups who can start medication within 48h (oseltamivir) or 36h (zanamivir) of exposure. Residential/nursing home residents may be offered prophylaxis regardless of vaccination status. Baloxavir marboxil is licensed but is not recommended by NICE. The UKHSA reserves its use for second-line prophylaxis if the viral strain is proven to be oseltamivir-resistant; it is unlikely we’ll prescribe it! Antivirals carry risk of harms and are of limited benefit in otherwise-healthy adult populations.

Useful resources: Websites (all resources are hyperlinked for ease of use in Red Whale Knowledge) Immunisation: England: Annual flu programme Northern Ireland: Influenza programme Scotland: NHS Scotland Winter programme 2025 – seasonal flu and COVID-19 vaccination Wales: The national influenza immunisation programme 2025 to 2026 (WHC/2025/020) UKHSA guideline (2025): treatment and prophylaxis of influenza