Flu shots are very much like seasonal fashion trends—every year, they get updated to match what’s "in" (or in this case, what’s infecting the most people). But unlike fashion, flu vaccines come in two versions—one for the Northern Hemisphere and another for the Southern Hemisphere.

If you've ever wondered why we need two different flu vaccines, which one Malaysians should get, who needs both, and why we need flu shots every year, this article will clear up the confusion.

1. Why Do We Need Two Versions of the Flu Vaccine?

The flu virus mutates constantly, which is why every year, we need a new vaccine to keep up. The problem is that flu seasons don’t happen at the same time everywhere.

The Northern Hemisphere flu season peaks between November and March, while the Southern Hemisphere flu season occurs from May to September.

Because of this, the World Health Organization (WHO) holds two major meetings each year to analyze global flu patterns and recommend vaccines. In February, they decide on the strains for the Northern Hemisphere vaccine, and in September, they do the same for the Southern Hemisphere vaccine.

Each version is specifically designed to protect against the most dominant flu strains in that region. That…is why we need two.

2. Why Do We Need Flu Shots Every Year?

Unlike vaccines for diseases like measles or hepatitis B, which provide long-term immunity, the flu vaccine needs to be updated and taken annually for two key reasons:

A. The Flu Virus is a Fast Mutator

Flu viruses change rapidly through a process called antigenic drift. This means that the flu strains circulating this year are likely to be different from those in previous years. Even if you got a flu shot last year, it may no longer provide strong protection against the current strains.

B. Immunity from the Flu Vaccine Wears Off

Even if the flu virus stayed the same (which it doesn’t), the protection from a flu shot fades over time. Studies show that the effectiveness of the flu vaccine starts to decline after about six months, which is why annual vaccination is necessary to maintain strong immunity.

Simply put, the flu vaccine is like an annual software update—you need the latest version to stay protected against the newest flu threats.

3. Which Flu Vaccine Should Malaysians Get?

Since Malaysia is near the equator, we don’t have "winter" or "summer" like other countries. Instead, flu spreads all year round, with two common peaks: May–July and November–January.

Because our flu patterns are more similar to Australia (our closest temperate neighbor), Malaysia generally follows the Southern Hemisphere flu vaccine. Most Malaysian clinics and pharmacies stock the Southern Hemisphere flu vaccine, which is released around April.

This version aligns better with Malaysia’s peak flu periods and provides longer-lasting protection against the strains likely to circulate here. If you miss the April update and are planning to get a flu shot later in the year (e.g., October–December), the Northern Hemisphere version may be the only one available. In that case, it is still better to get vaccinated than not at all.

4. Who Needs Both Flu Vaccines?

For most Malaysians, one flu shot a year (preferably the Southern Hemisphere version) is enough. However, some people who travel frequently between hemispheres may need both versions.

Who should consider getting BOTH flu shots in a year?

Frequent travelers between Malaysia and the Northern Hemisphere (e.g., the U.S., Europe, Japan, China) – If you got the Southern Hemisphere shot in April, but you’re traveling to the U.S. in December, the flu strains there might be different. In this case, you may need a booster with the Northern Hemisphere vaccine before your trip.

Students studying abroad (especially in Europe, the U.K., or the U.S.) – If you’re going to university overseas, check which flu vaccine is used there and get vaccinated accordingly.

High-risk individuals (e.g., elderly, people with chronic illnesses, pregnant women) who travel frequently – If you have a weaker immune system and travel between hemispheres, getting both flu shots can provide extended protection.

Healthcare workers or people frequently exposed to travelers – If you work in an airport, hospital, pharmacy or international setting, consider getting both vaccines for extra protection.

To summarize all the above…

If you’re a Malaysian and don’t plan to travel much, get the Southern Hemisphere flu vaccine. However, availability can vary, and getting any updated flu vaccine (NH or SH) is preferable to remaining unvaccinated if the recommended SH version is not in stock. 

If you’re traveling to a country in the Northern Hemisphere (Japan, China, Korea, Taiwan etc) during flu season (October–March), consider getting a second flu shot before your trip.

If you’re a frequent traveler hopping between hemispheres, talk to your pharmacist or doctor about whether you need BOTH vaccines.

At the end of the day, the most important thing isn’t which version you get, but making sure you’re protected from flu when it matters most. Stay safe, stay healthy, and don’t let the flu ruin your plans.