Flu season: when it starts in different countries, and why

Flu season depends on where you live. In the United States the CDC says it usually occurs in the fall and winter, and that most of the time flu activity peaks between December and February. That is a statement about one country in the Northern Hemisphere. Move south of the equator, or toward it, and the calendar changes, sometimes by half a year and sometimes into no clear calendar at all.

This article puts four sources side by side to see what they say about timing. It is informational material, not medical advice. If symptoms are severe, getting worse, or not going away, that is a question for a doctor.

Flu season months: the short answer

For the Northern Hemisphere there is an official sentence to quote. The CDC writes: "In the United States, flu season usually occurs in the fall and winter," and "most of the time flu activity peaks between December and February" (CDC). It describes the US only.

For the Southern Hemisphere there is no month range in the sources used here, and this article will not invent one. What can be said is a calendar fact plus a research finding. Winter in the Southern Hemisphere is June to August. A global analysis of 246 sites found clear flu epidemics in winter months at most temperate sites (Li et al., 2019), and a review of 137 locations found that flu and RSV consistently peaked in winter in temperate places (Bloom-Feshbach et al., 2013). Put together: in temperate places south of the equator the expected flu season is local winter, not the same months as in Europe or North America. Whether a particular year follows that is a separate question, and the sources do not answer it.

Why is flu season in the winter?

Short version: at least one large comparison ties it to cold, dry air, but only in part of the world. The 78-site study by Tamerius and colleagues found two types of conditions linked to seasonal flu epidemics, which they call "cold-dry" and "humid-rainy". Where monthly average specific humidity or temperature falls below roughly 11 to 12 g/kg and 18 to 21 °C during the year, flu peaks in the cold, dry season, which is winter. Where they never fall below those levels, the peak is more likely to come in the months with the most rain, above 150 mm a month (Tamerius et al., 2013).

That is an association between climate and the timing of epidemics, not a measurement of why a given person catches flu. The mechanisms researchers have proposed, from virus survival in dry air to people spending more time indoors, are covered in why we get sick more in winter. This article stays with the calendar.

Flu season in the tropics

Closer to the equator the pattern loosens. In the Lancet analysis, flu had clear winter epidemics at most temperate sites, but the timing of epidemics was more variable and less seasonal with decreasing distance from the equator. The length of an epidemic differed too: 3.8 months (95% CI 3.6 to 4.0) at temperate sites and 5.2 months (4.9 to 5.5) in the tropics. Here an epidemic means the smallest number of months that account for 75% of a year's positive samples (Li et al., 2019).

The 137-location review adds two details. Several temperate locations had flu activity twice a year, with peaks in winter and in summer, and this semi-annual pattern was relatively common in the tropics of Southeast Asia. Still, 50% of tropical locations had a distinct flu season lasting up to six months (Bloom-Feshbach et al., 2013). And Tamerius's paper notes that in tropical and subtropical regions epidemics often occur during the rainy season or run year-round without a well-defined season.

So "flu season in the tropics" is not one thing. In some places it is a rainy-season wave, in some a pair of peaks, in some a long, blurred stretch. A single month range would be false precision.

Flu and RSV keep different calendars

Respiratory syncytial virus (RSV) is another common winter respiratory virus, and the Lancet analysis compared it with flu. Flu had clear seasonal epidemics at most temperate sites but was less seasonal near the equator. RSV, unlike flu, had clear seasonal epidemics in both temperate and tropical regions. In the tropics of each hemisphere it started in late summer months, and it reached most temperate sites in winter. At most temperate sites flu came later than RSV, by 0.3 months (95% CI -0.3 to 0.9), a gap whose interval includes zero. No clear order was seen in the tropics (Li et al., 2019).

For parainfluenza the same analysis found epidemics mostly in spring and early summer in each hemisphere. The practical point for a reader is modest: "the winter bug" is several different viruses, and they do not all share one season.

What nobody knows yet

• The months of flu season in a specific Southern Hemisphere country. The sources here give "winter" for temperate sites, and the calendar says winter there is June to August. They do not give a country-by-country table, and none is quoted.
• Where the next season will start. The CDC says activity "usually" occurs in fall and winter and "most of the time" peaks between December and February. Both phrases allow years that do not fit.
• How the picture looks in Africa and Central America. Bloom-Feshbach's authors write that information on seasonal patterns remains limited in large undersampled regions, including these two.
• Why the tropics differ. Tamerius links tropical epidemics to rainy seasons, but the authors describe a model fitted to climate and epidemiological data from 78 sites. It shows a statistical association, not a mechanism.
• What any of this means for you personally. The data are monthly counts of laboratory-confirmed infections across populations. They say when the virus is circulating, not whether you will get sick.

The Lancet authors conclude that information about seasonality matters for health services planning and for influenza vaccination strategy. That is their conclusion about planning at the level of health systems, and this article gives no advice on vaccination.

What you can look at in your own data

The app records local outdoor weather (pressure, temperature, relative humidity, wind speed, UV index) next to your own symptom entries and Apple Health data, and looks for correlations in your own history with Pearson correlation, ANOVA and false-discovery-rate correction. Its nine fixed symptoms are headache, fatigue, drowsiness, dizziness, anxiety, irritability, nausea, joint pain and insomnia. Cough and fever are not among them, and the app does not recognize, diagnose or predict infections.

So it cannot tell you when flu is going around in your city. What it can show is whether, in your entries, a low-energy week lined up with a cold, dry spell outdoors. A pattern like that needs care before it is called a cause; the reasons are in weather sensitivity: what the science says. For the wider picture of why some diseases have a calendar at all, see seasonal diseases.