Why the flu vaccine is needed every year: explainer based on US CDC information
As flu season approaches, the health portal “Health”, citing the US Centers for Disease Control and Prevention, explains why flu vaccination must be repeated annually. The reasons are the virus's ability to change and the gradual weakening of vaccine protection.
Ahead of the flu season, many people wonder why vaccination against influenza has to be repeated every year and whether a shot received last season still protects them. The health portal “Health”, citing the US Centers for Disease Control and Prevention (CDC), explains why the vaccine formula is regularly reviewed, when it is best to get vaccinated and what the shot actually does.
The virus changes
As the influenza virus spreads and multiplies, it undergoes small genetic changes. Science calls this process antigenic drift. As a result, the virus's surface structures can change, and the immune system may find it harder to recognise new variants. That is why a person can catch flu several times, even within a single season.
Every year, forecasts are made about which virus strains may spread in the coming season, and the vaccine for that season is developed accordingly. An international network of experts monitors the spread of viruses worldwide, analyses the data and recommends which strains to include. The composition is set in good time so manufacturers can prepare the vaccines. For this reason, last year's vaccine is not a full substitute for the new one.
Why flu is dangerous
Influenza is a highly contagious respiratory infection spread through droplets and aerosols. It can cause fever, cough, muscle pain and marked weakness, and sometimes severe complications, including pneumonia. The risk is especially high for small children, older people, pregnant women and people with certain chronic conditions. Infants under six months are not vaccinated against flu, so vaccination of those around them matters.
Vaccination does not guarantee that a person will not fall ill, but it can reduce the risk of severe illness and complications, and the disease may be milder.
When to get vaccinated
For most people, September and October are considered the best months, but vaccination is possible later if viruses keep circulating. The immune response usually takes about two weeks to develop. In Latvia, timing should follow the advice of local health care providers and the Centre for Disease Prevention and Control.
How long the shot works
Protection can last several months but tends to weaken over time, depending on age, health and the strains in circulation. Injectable seasonal flu vaccines contain no virus capable of causing influenza. Soreness at the injection site, tiredness, headache or a slight fever are usually mild and pass quickly. Illness after vaccination is still possible, since effectiveness depends on how well the vaccine matches circulating strains and on the person's own immune response.
