Annual burden of influenza globally and in the U.S., how seasonal flu vaccines are formulated each year, measured effectiveness data by season, and high-risk population guidance.
Influenza is a contagious respiratory illness caused by influenza viruses that infect the nose, throat, and sometimes the lungs. It causes mild to severe illness and can lead to death.
Before the availability of influenza vaccines, seasonal flu epidemics caused significant mortality, particularly during pandemics.
CDC. "Flu Symptoms & Complications."
Several types of influenza vaccines are available, updated annually to match circulating strains.
Protects against 4 influenza strains (2 A, 2 B). Standard dose for adults 18-64.
Nasal spray vaccine for non-pregnant individuals ages 2-49.
For adults 65+. Enhanced immune response in older adults.
| Season | Overall Effectiveness | Match to Circulating Strains |
|---|---|---|
| 2023-2024 | 36-52% | Good match |
| 2022-2023 | 28-37% | Moderate match |
| 2021-2022 | 31-35% | Good match |
| 2020-2021 | 38-49% | Very good match |
| 2019-2020 | 39% (lower for B) | Mixed match |
Source: CDC. "Seasonal Flu Vaccine Effectiveness Studies."
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