Influenza

What Is the Flu?

“Flu” (influenza) is a contagious respiratory illness caused by influenza viruses that infect the nose, throat, and sometimes the lungs.
It can range from mild to severe, and sometimes can lead to hospitalization or death, especially in vulnerable groups.
Common symptoms include fever (or feeling feverish/chills), cough, sore throat, runny or stuffy nose, muscle or body aches, headaches, fatigue, sometimes vomiting or diarrhea (especially in children)
The onset is usually sudden — you may feel well one day, then become very ill within 1–3 days of exposure.

- Influenza spreads mainly when infected people cough, sneeze, or talk, releasing tiny droplets that others inhale or that land on surfaces.
- You can get it by touching a surface or object with the virus on it, then touching your mouth, nose, or eyes.
- There are several types of influenza viruses: A, B, C, and D. Only types A and B cause the seasonal epidemic flu in humans.
- Influenza A viruses are subtyped (for example, H1N1, H3N2) and can infect humans, animals, or both.
- The viruses evolve over time via antigenic drift (small mutations) and antigenic shift (sudden large changes) — drift causes seasonal changes; shift can produce new viral strains that humans have little immunity to.

- Influenza is not new; flu pandemics have occurred at various times in history (e.g., 1918 “Spanish flu,” 1957, 1968, 2009 H1N1).
- Many seasonal flu strains derive from viral lineages that circulate among animals (birds, swine) and periodically cross over into humans or reassort.
- Ongoing surveillance, genome sequencing, and virus evolution studies help track the origin, evolution, and spread of influenza strains (for example, the Influenza Genome Sequencing Project)

In the Northern Hemisphere (including the U.S.), flu season typically spans October through May.
Flu activity tends to peak between December and February, though sometimes it peaks earlier or later.
In the Southern Hemisphere, flu season is generally during their winter months (roughly April–September). Surveillance in the Southern Hemisphere is often used to anticipate which strains might circulate in the upcoming Northern Hemisphere season.
In the U.S., for example, the 2024–2025 flu season began in October 2024, with cases rising above baseline by late December, a peak in early 2025, and gradual decline by April.

Annual flu vaccination: the foundational prevention strategy
Hand hygiene: frequent handwashing with soap and water or using hand sanitizer
Avoid close contact with sick people; avoid touching face (mouth, nose, eyes) with unwashed hands
Cover coughs and sneezes with a tissue or your elbow; dispose of tissues properly
Clean and disinfect surfaces (doorknobs, phones, etc.) that may have the virus
Good ventilation- letting fresh air circulate reduces risk of airborne transmission
Stay home when sick to avoid spreading it to others.

Flu vaccines are updated yearly to match the strains predicted to circulate. The vaccine often includes 3 or 4 strains (trivalent or quadrivalent) chosen by global surveillance bodies.
Effectiveness is variable. It depends on:
- How well the vaccine strains match the circulating viruses
- The age and health of the person receiving it
- The time since vaccination (immunity wanes)
- The study design and outcome measured (e.g. preventing any infection vs. preventing hospitalization)

Over many seasons, adjusted vaccine effectiveness has ranged widely.
For example, in past seasons, flu vaccination has prevented millions of illnesses, doctor visits, hospitalizations, and deaths in the U.S.
In 2025 interim data from Southern Hemisphere countries indicate vaccines offered important protection against outpatient illness and hospitalization, though uptake was modest (21% among patients with influenza-like illness in one dataset)
A limitation: immunity tends to decline with time after vaccination. Some studies show measurable waning starting about 6 weeks (≈41 days) post vaccination, with percent protection declining monthly thereafter, especially in older adults.
Despite less than perfect efficacy, vaccination is still a critical tool because it reduces severity, prevents complications, and contributes to population-level protection (herd effect).

Surveillance & Strain Selection: Scientists globally monitor circulating influenza strains (via WHO, CDC, etc.). Based on that, they predict which strains to include in the next season’s vaccine. This process must occur months before the next season begins.
Vaccine Technology Innovation: Research pushes for better vaccine platforms (e.g. cell-based, recombinant, universal vaccines) that may offer broader or longer-lasting protection.
Effectiveness & Waning Studies: Ongoing clinical and observational studies help quantify how effective vaccines are in real-world settings and how rapidly protection wanes — which informs recommendations on timing, booster strategies, or vaccine design.
Antigenic Evolution Studies: Research into viral mutation, antigenic drift/shift, and molecular evolution helps us anticipate possible “escape” mutants and adjust vaccine strategy accordingly.
Clinical Trials: Testing new vaccine formulations, adjuvants (immune boosters), delivery methods (e.g. nasal spray, high-dose, universal vaccine candidates), or antiviral drugs in controlled trials is essential to validate safety and effectiveness before widespread use.
Data-driven public health policy: Modeling, forecasts, and real-time flu analytics enable health authorities to allocate resources, time vaccination campaigns, and issue public health guidance.