Influenza

An evolving virus. A year-round challenge.

Explore how influenza changes, why vaccines are regularly updated, and what science is uncovering about broader, longer-lasting protection.

About this guide

A guide to influenza: the virus, how it changes, what that does to vaccines, and what the evidence actually shows. Every figure on this page links to the document it came from.

Understanding influenza

Four virus types, two of which matter for seasonal epidemics

Influenza is an acute respiratory infection caused by influenza viruses. It is not “a bad cold”: it has a distinct virus, a distinct seasonal pattern and a measurable burden of hospitalization and death.

The four types at a glance

Behind seasonal epidemics

The two types that drive seasonal flu in people.

Influenza A
Seasonal flu. Pandemic potential.
- A(H1N1)
- A(H3N2)
The only type known to have caused pandemics.
Influenza B
Seasonal flu.
- B/Victoria
B/Victoria circulates. B/Yamagata has not been detected since March 2020.

Outside seasonal epidemics

A different role in human health.

Influenza C
Usually mild illness
Does not cause epidemics.
Influenza D
Primarily affects cattle
Not known to cause illness in people.

Explore the four types

Influenza A
Subtyped by two surface proteins, hemagglutinin (H) and neuraminidase (N): 18 H and 11 N subtypes are known. The two subtypes now circulating routinely in people are A(H1N1) and A(H3N2). Only influenza A viruses are known to have caused pandemics.
Influenza B
Not divided into subtypes but into two lineages, B/Victoria and B/Yamagata. B/Yamagata viruses have not been detected by global surveillance after March 2020 (see Science and discovery).
Influenza C
Usually causes mild illness and is not considered a public-health problem; it does not cause epidemics.
Influenza D
Primarily infects cattle and is not known to infect or cause illness in people. It was first isolated from pigs in Oklahoma in 2011 and has seven genome segments.

Influenza A and B viruses carry their genome as eight separate segments of RNA rather than one continuous strand. The two proteins studding the outside — hemagglutinin, which binds the virus to cells, and neuraminidase, which releases new particles — are what the immune system mostly “sees” and what the vaccine mostly targets.

The virus spreads easily from person to person, mainly in infectious droplets when people cough or sneeze, and indirectly through respiratory secretions on hands and surfaces; aerosol transmission also plays a role. The incubation period is about two days, ranging from one to four.

Virus is found in nose and throat samples from about one day before symptoms start to five to seven days after — so people are infectious before they know they are ill.

Complications

Most people recover within a week without medical care, but severe cases can involve pneumonia, myocarditis (inflammation of the heart muscle) or encephalitis, and secondary bacterial pneumonia is a typical complication. The risk of severe disease is highest in older people and in those with chronic conditions.

Sudden onset of fever
Cough, usually dry, which can last two weeks or more
Headache
Muscle and joint pain
Severe malaise (feeling unwell)
Sore throat and runny nose

Pregnant women
Children under five
Older people
People with chronic medical conditions
People with weakened immune systems
Health workers (higher exposure, and risk of onward transmission)

Influenza, the common cold and COVID-19

This table is about biology, not diagnosis. Symptom overlap between these infections is large, and the only way to tell them apart in an individual is a laboratory test.

	Influenza	Common cold	COVID-19
Cause	Influenza A and B viruses	Many different viruses (rhinoviruses, common-cold coronaviruses and others)	SARS-CoV-2
Onset	Typically sudden, with fever	Typically gradual	Variable
Seasonal pattern in temperate regions	Winter epidemics	Year-round, more in colder months	Waves not confined to winter
Vaccine	Yes — composition reviewed twice a year by WHO	No	Yes — composition also periodically updated

Signature module

Why does the flu vaccine need updating?

Seven connected diagrams, from the virus particle to the vaccine calendar. Step through them in order, switch between plain language and scientific detail, and open the sources behind any statement. Nothing in this module is a simulation of real numbers: where a picture is conceptual, it is labeled as such.

Step 1 of 7

The parts that matter for infection, immunity and evolution

An influenza particle is a small envelope studded with two kinds of protein spikes and carrying its genetic instructions inside as eight separate pieces. Select a part to see what it does and why it matters for vaccines.

How to read these diagrams: they show mechanisms, not measurements. Shapes and distances are chosen for clarity, not drawn to scale, and no diagram predicts how well a particular vaccine will work in a particular person.

Select a structure

Hemagglutinin (HA)

Binds the virus to sialic-acid receptors on the cell surface and drives fusion with the cell membrane. Gives influenza A its “H” number.

The main target of neutralizing antibodies and the main site of antigenic drift. The head domain changes far more than the stem.

Putting the module together: influenza viruses accumulate genetic change continuously, and some of that change alters what antibodies recognize (drift). Several types and subtypes circulate at once, with a shifting balance across regions and months. Vaccine composition has to be recommended months before the season it serves, on the basis of what is circulating at the time — so it is a forecast, and forecasts sometimes miss. Meanwhile, the protection a vaccine gives fades over months and depends on age, prior exposure and subtype.

That is why the composition is reviewed every year and updated when needed, and why annual vaccination is recommended even in years when nothing changes — two different reasons that are often conflated. A vaccine that made both unnecessary would have to aim at the conserved parts of the virus; that work is under way and, as of September 2026, still in early clinical stages.

Vaccines and treatments

Several technologies, one recommendation, a menu of antivirals

All statements about approvals and recommendations here refer to the United States for the 2026–27 season unless stated otherwise, because that is where the most complete public documentation exists. Products and age indications differ in the EU and elsewhere.

Egg-based inactivated (IIV3)
Candidate vaccine viruses are grown in embryonated hens’ eggs, inactivated and purified. The oldest and still the most widely used method; an inactivated vaccine was licensed for civilian use in 1945.
US 2026–27: Fluarix, FluLaval (GSK); Fluzone and Fluzone High-Dose (Sanofi). Ages 6 months and over; High-Dose 65 and over.
Growing viruses in eggs can introduce egg-adapted changes that make the vaccine virus differ from circulating viruses; a 2017 study attributed reduced 2016–17 effectiveness against A(H3N2) in part to a glycosylation site absent from the egg-adapted strain.
Cell-culture inactivated (ccIIV3)
Viruses are grown in mammalian cell lines instead of eggs; since 2019–20 all candidate viruses used in Flucelvax are cell-derived, avoiding egg adaptation.
US 2026–27: Flucelvax (Seqirus), ages 6 months and over.
CDC notes cell-based production might allow faster start-up in a pandemic because cells can be banked and frozen; the page gives no timeline comparison.
Recombinant (RIV3)
The HA protein is produced directly by recombinant methods; no candidate vaccine virus is needed and there is no egg adaptation.
US 2026–27: Flublok (Sanofi), ages 9 and over.
Preferentially recommended, alongside high-dose and adjuvanted vaccines, for adults 65 and over in US guidance.
Adjuvanted (aIIV3)
An egg-based inactivated vaccine formulated with an adjuvant to strengthen the immune response in older adults.
US 2026–27: Fluad (Seqirus), ages 65 and over.
One of the three preferentially recommended options for adults 65 and over.
Live attenuated (LAIV3)
Weakened viruses given as a nasal spray.
US 2026–27: FluMist (AstraZeneca), ages 2 through 49.
Not for people 50 and over or under 2 in the US.
mRNA (new in 2026)
mRNA encoding HA is delivered in lipid nanoparticles, as in COVID-19 vaccines.
US: mFLUSIVA (Moderna), approved 5 August 2026 for adults 50 and over; company states it expects availability for the 2026–27 season. Under review in the EU, Canada and Australia (company-reported).
Not yet listed on CDC’s 2026–27 vaccine summary as of 1 September 2026. Approval for those 65 and over is accelerated and contingent on post-marketing evidence.

For 2026–27, WHO recommended trivalent vaccines containing A/Missouri/11/2025 (H1N1)pdm09-like, A/Darwin/1454/2025 (H3N2)-like and B/Tokyo/EIS13-175/2025 (B/Victoria)-like viruses for egg-based products, with different reference viruses for cell-, recombinant- and nucleic-acid-based products. All US vaccines have been trivalent since 2024–25, when the B/Yamagata component was removed.

How well vaccines work is measured after the fact

CDC estimates effectiveness each season with a test-negative design: among people seeking care for respiratory illness, it compares vaccination rates in those who test positive for influenza with those who test negative, adjusting for site, age, sex, underlying conditions and time from onset to enrollment.

Adjusted vaccine effectiveness against medically attended influenza, United States, by season

VE Preliminary 95% CI 95% CI ∋ 0┆ Not estimated by the source

Units: percentage reduction in the odds of laboratory-confirmed influenza among vaccinated versus unvaccinated outpatients, all ages and all virus types combined. Bars show the 95% confidence interval.

Scope: US Flu VE Network sites (Wisconsin alone in 2004–05; up to eight states in recent seasons). 2020–21 not estimated. 2024–25 and 2025–26 are preliminary and come from a larger network, which is why their intervals are much narrower.

Method: test-negative case-control design; estimates adjusted for study site, age, sex, underlying medical conditions and days from illness onset to enrollment. A confidence interval crossing zero means the data are consistent with no measurable effect that season. Effectiveness against hospitalization is estimated separately and is not shown.

Source page dated 16 March 2026; transcribed 12 September 2026.

Source

Antiviral treatment

Four antivirals are FDA-approved and recommended by CDC (page dated 10 March 2026). CDC recommends starting treatment as soon as possible for hospitalized patients, those with severe or progressive illness and those at higher risk; for otherwise healthy outpatients, within two days of onset. EU indications differ.

Oseltamivir (Tamiflu)
Neuraminidase inhibitor
Oral
Treatment from 14 days; prophylaxis from 1 year (US label)
Zanamivir (Relenza)
Neuraminidase inhibitor
Inhaled
Treatment from 7 years; prophylaxis from 5 years; not for people with underlying airway disease
Peramivir (Rapivab)
Neuraminidase inhibitor
Intravenous
Treatment from 6 months; not recommended for prophylaxis
Baloxavir marboxil (Xofluza)
Cap-dependent endonuclease (polymerase) inhibitor
Oral, single dose
Treatment and post-exposure prophylaxis from 5 years

CDC describes resistance among circulating viruses as currently very low but able to change; reduced susceptibility has emerged in immunocompromised patients and in children treated with baloxavir. WHO’s February 2026 report found no evidence of reduced baloxavir susceptibility among the B/Victoria viruses examined.

Science and discovery

Findings that changed how influenza is understood

Each card states what was found, why it matters for vaccines, and what the finding cannot tell us. The evidence label is the page’s own judgment of the source’s strength.

Established finding
A whole lineage of influenza B appears to have vanished
No confirmed detections of circulating B/Yamagata lineage viruses have been recorded anywhere in global surveillance after March 2020.
Established finding
Antigenic change is jumpier than genetic change
By placing viruses on an “antigenic map” from ferret antisera data, Smith and colleagues showed in 2004 that A(H3N2) evolved antigenically in clusters, with punctuated jumps, while genetic change was gradual; some single substitutions had a disproportionately large antigenic effect.
Established finding
Your first childhood flu shapes your defenses for life
Using all known human cases of H5N1 and H7N9, Gostic and colleagues found that a person’s first influenza A infection confers lifelong protection against severe disease from novel HA subtypes in the same phylogenetic group: 75% against severe infection and 80% against death.
Established finding
Eggs can quietly change the vaccine virus
Contemporary A(H3N2) viruses carry a glycosylation site in antigenic site B that the egg-adapted 2016–17 vaccine strain lacked; antibodies raised by that egg-grown strain poorly neutralized the glycosylated circulating virus.
Established finding
Protection from a seasonal vaccine fades within the season
In US network data pooled over 2011–12 to 2014–15, effectiveness fell by about 7 percentage points per month for A(H3N2) and B and 6–11 per month for A(H1N1)pdm09, while remaining above zero for at least five to six months.
Established finding
A fourth influenza type, found in pigs
In 2011 a novel orthomyxovirus with seven RNA segments, about 50% amino-acid identity to human influenza C, was isolated from pigs in Oklahoma; it is now classified as influenza D and is mainly found in cattle.
Emerging evidence
Stem-directed antibodies can be raised in people
A first-in-human phase 1 trial of a group 2 HA stabilized-stem ferritin nanoparticle vaccine (25 adults) was safe and induced stem-binding and neutralizing antibodies that stayed above baseline at 40 weeks, including cross-reactive H3 and H7 binding.

Milestones

An H1N1 virus with genes of avian origin infected about 500 million people — a third of the world’s population — and killed at least 50 million worldwide, about 675 000 in the United States (historical estimates).

Market and industry

What can be verified about the influenza vaccine market

This section reports only what audited filings, company results announcements and regulatory documents say. Commercial market-size forecasts are not reproduced: their methods are opaque and they are not evidence.

Established finding
Who makes the vaccines used in the United States
For 2026–27, CDC lists products from four manufacturers: GSK (Fluarix, FluLaval), Sanofi (Fluzone, Fluzone High-Dose, Flublok), Seqirus (Flucelvax, Fluad) and AstraZeneca (FluMist). Moderna’s mFLUSIVA was approved on 5 August 2026 and is not yet on that list as of 1 September 2026.
Established finding
Sanofi: influenza and COVID-19 vaccines, 2025
Company-reported: full-year 2025 sales of the “Influenza, COVID-19” vaccines line were €2 314 million, down 5.8% at constant exchange rates, of which €1 328 million in the United States. Fourth-quarter sales rose 31.5% to €575 million, which Sanofi attributed to more circulating virus and a higher vaccination rate than anticipated. The line combines influenza and COVID-19 products; an influenza-only figure is not disclosed.
Established finding
CSL Seqirus: fiscal 2025 and a planned demerger
Company-reported, year to 30 June 2025: CSL Seqirus revenue US$2 166 million, up 2% at constant currency; Fluad sales US$901 million (down 14%) and Flucelvax US$474 million (down 12%), which CSL attributed to significantly lower immunization rates, particularly in the US, partly offset by about US$150 million of H5 avian-influenza pandemic-preparedness contracts. CSL announced its intention to demerge Seqirus as an ASX-listed company before the end of its fiscal 2026.
Emerging evidence
Technology trend: away from eggs, and now mRNA
Cell-based, recombinant and adjuvanted products carry US preferential recommendations for older adults, and the first mRNA seasonal vaccine was approved in August 2026. Whether these shift market share is not something this page can verify from public filings, and it does not claim to.
Evidence gap
Independent market sizing
No independent, methodologically transparent estimate of the global influenza vaccine market was identified among the sources reviewed for this page. Figures circulated by commercial research firms are proprietary and were not used.

Company figures are as reported by the companies, in their own segment definitions and currencies, and are not audited by this page. Sanofi reports in euros at constant exchange rates; CSL in US dollars at constant currency; the two are not comparable.

Methodology

How this page was made

Source selection. Factual statements are supported by, in order of preference: WHO and national public-health agencies (CDC, ECDC); regulatory documents; peer-reviewed research identified by DOI; audited company filings and results announcements for market information; and trade press only for reporting of very recent events where the primary document was not yet indexed. Each source was opened and read on 12 September 2026, and the date shown next to it is the source’s own publication or last-updated date.

Data transformations. None. The two charted series (CDC US burden by season; CDC US vaccine effectiveness by season) were transcribed from the published figure and table respectively, without interpolation, smoothing or combination with other datasets. Seasons the source does not estimate (2020–21) are stored as missing and drawn as a gap. Values are point estimates; CDC publishes uncertainty intervals for burden in its full tables, which are not reproduced in the summary figure used here, so the burden chart shows no intervals. The effectiveness chart shows the 95% confidence intervals exactly as published.

Assumptions and limitations. US data are used for charts because they are the most complete open, season-by-season series with stated methods; they do not represent Europe or the world. Surveillance intensity differs between countries, which is why no map is drawn. Vaccine effectiveness estimates are population statistics from specific study designs and are not individual predictions. The interactive diagrams are conceptual and are labeled as such; the vaccine-match explorer only surfaces quoted findings and does not compute anything.

Evidence labels. “Established finding” is used for results published in peer-reviewed journals or by public-health agencies and not, to the page authors’ knowledge, contradicted by later work. “Modeled estimate” marks numbers produced by statistical models rather than counts. “Emerging evidence” marks early-phase or single-study results. “Forecast or scenario” marks statements about the future. “Evidence gap” marks where reliable information was sought and not found.

Update policy. Statements about approvals, recommendations and products are dated and jurisdiction-specific; they will go out of date. The page should be re-checked before each northern-hemisphere season, at minimum against the WHO February recommendation and CDC’s September vaccine summary.

References

Every source used on this page

Listed in the order they appear in the source list, with publisher, publication or update date and the kind of evidence each represents.

[1]Public-health or regulatory agency

Influenza (seasonal) — fact sheet

World Health Organization

Published / updated:: February 28, 2025; Accessed 12 Sep 2026

An evolving virus. A year-round challenge.

About this guide

Four virus types, two of which matter for seasonal epidemics

Influenza A

Influenza B

Influenza C

Influenza D

Influenza A

Influenza B

Influenza C

Influenza D

Complications

Influenza, the common cold and COVID-19

Why does the flu vaccine need updating?

The parts that matter for infection, immunity and evolution

Hemagglutinin (HA)

Large numbers, mostly modeled

Worldwide (WHO)

European Union / EEA (ECDC)

Estimated influenza burden in the United States, by season

A world map is deliberately missing

Several technologies, one recommendation, a menu of antivirals

Egg-based inactivated (IIV3)

Cell-culture inactivated (ccIIV3)

Recombinant (RIV3)

Adjuvanted (aIIV3)

Live attenuated (LAIV3)

mRNA (new in 2026)

How well vaccines work is measured after the fact

Adjusted vaccine effectiveness against medically attended influenza, United States, by season

Antiviral treatment

Oseltamivir (Tamiflu)

Zanamivir (Relenza)

Peramivir (Rapivab)

Baloxavir marboxil (Xofluza)

Findings that changed how influenza is understood

A whole lineage of influenza B appears to have vanished

Antigenic change is jumpier than genetic change

Your first childhood flu shapes your defenses for life

Eggs can quietly change the vaccine virus

Protection from a seasonal vaccine fades within the season

A fourth influenza type, found in pigs

Stem-directed antibodies can be raised in people

Milestones

What can be verified about the influenza vaccine market

Who makes the vaccines used in the United States

Sanofi: influenza and COVID-19 vaccines, 2025

CSL Seqirus: fiscal 2025 and a planned demerger

Technology trend: away from eggs, and now mRNA

Independent market sizing

Five questions

How this page was made

Every source used on this page

Influenza (seasonal) — fact sheet

Types of influenza viruses

How flu viruses can change: “drift” and “shift”

Selecting viruses for the seasonal influenza vaccine

Key facts about seasonal flu vaccine

CDC seasonal flu vaccine effectiveness studies (past seasons’ estimates)

About estimated flu burden — estimated US influenza burden by season, 2010–2025

Different types of flu vaccines

Interim clinical considerations for the use of seasonal influenza vaccines in the United States, 2026–27

Trivalent influenza vaccines

Cell-based flu vaccines

Influenza antiviral medications: summary for clinicians

1918 pandemic (H1N1 virus)

2009 H1N1 pandemic (H1N1pdm09 virus)

Influenza historic timeline, 1930 and beyond

Recommended composition of influenza virus vaccines for use in the 2026–2027 northern hemisphere influenza season (full report)

Recommendations announced for influenza vaccine composition for the 2025–2026 northern hemisphere influenza season

Recommendations announced for influenza vaccine composition for the 2024–2025 northern hemisphere influenza season

WHO recommendations on the composition of influenza virus vaccines (index)

FluNet — global influenza surveillance data

Factsheet about seasonal influenza

Variable influenza vaccine effectiveness by subtype: a systematic review and meta-analysis of test-negative design studies. Lancet Infect Dis 2016;16(8):942–951

Intraseason waning of influenza vaccine protection: evidence from the US Influenza Vaccine Effectiveness Network, 2011–12 through 2014–15. Clin Infect Dis 2017;64(5):544–550

Mapping the antigenic and genetic evolution of influenza virus. Science 2004;305(5682):371–376

Potent protection against H5N1 and H7N9 influenza via childhood hemagglutinin imprinting. Science 2016;354(6313):722–726

Contemporary H3N2 influenza viruses have a glycosylation site that alters binding of antibodies elicited by egg-adapted vaccine strains. Proc Natl Acad Sci USA 2017;114(47):12578–12583

Isolation of a novel swine influenza virus from Oklahoma in 2011 which is distantly related to human influenza C viruses. PLoS Pathog 2013;9(2):e1003176