Influenza, or the flu, is a viral infection that tends to spread during the rainy and winter seasons. It’s easily transmitted through the respiratory system, meaning anyone can catch it. However, certain groups are at higher risk—young children, the elderly, pregnant women, and people with chronic health conditions such as diabetes, heart disease, or asthma. While the flu may start off looking like a common cold, in severe cases it can progress to pneumonia, myocarditis (heart inflammation), or even encephalitis (brain inflammation).

One of the best ways to reduce the risk is to get the flu vaccine every year. Although the vaccine does not prevent infection 100%, it significantly lowers the chances of getting sick, reduces symptom severity, and helps prevent hospitalization or serious complications. The flu vaccine is made from inactivated or processed viruses that are harmless to the body but can safely and effectively stimulate immunity.

On average, flu vaccines are about 40–60% effective, depending on whether the circulating virus matches the strains covered by that year’s vaccine. This protection is strong enough to significantly reduce the risk of severe illness. The U.S. Centers for Disease Control and Prevention (CDC) reports that flu vaccination can reduce hospitalizations by up to 60% among children and older adults.

The reason flu vaccines must be given every year is because the virus mutates and changes strains frequently. The World Health Organization (WHO) continuously monitors global flu outbreaks and recommends new vaccine formulas annually. Current vaccines are designed to cover four strains of the flu virus—two from Type A and two from Type B—known as the quadrivalent vaccine.

There are two main formulations: the Northern Hemisphere and Southern Hemisphere vaccines. In Thailand, both types are available. The Southern Hemisphere vaccine is best suited for May–September, aligning with Thailand’s rainy season, while the Northern Hemisphere vaccine is used later in the year and is recommended for people traveling to Europe, the U.S., or Japan during winter.

Travelers should consider their destination when choosing the vaccine. For example, those going to Europe or North America in late year should get the Northern Hemisphere vaccine at least two weeks before travel. Meanwhile, travelers heading to Australia or South Africa between April and September should opt for the Southern Hemisphere version.

Flu vaccines are generally safe, though mild side effects may occur, such as pain, swelling, or redness at the injection site, fatigue, or a low-grade fever. These usually resolve within one or two days and are not dangerous.

Certain groups are strongly encouraged to get vaccinated:

• Children aged 6 months and older

• Older adults

• Pregnant women

• People with chronic conditions such as diabetes, heart disease, or lung disease

• Those with weakened immune systems

• Healthcare workers and people working in crowded environments

Another key reason for yearly vaccination is that immunity gradually decreases over time, usually lasting only 6–8 months. Renewing immunity each year ensures the body stays prepared against constantly evolving flu viruses.

In short, the flu vaccine is a small step with a big long-term payoff. It not only protects you from illness but also helps safeguard loved ones and eases the burden on public health systems. If you’re in a high-risk group, planning to travel, or working in high-exposure environments, consult your doctor and choose the right vaccine for the season and your destination. Just one shot a year can help build strong protection for yourself and those around you—but if you travel frequently or face higher risks, you may even need to consider getting vaccinated twice a year.