icddr,b scientists help reveal a striking geographical pattern of influenza virus epidemics in Bangladesh

Influenza virus epidemics impose a serious global public health burden, with up to 650,000 people dying of flu-related causes every year. In Bangladesh, an annual incidence of 458 cases per 100,000 people and accompanying morbidity and mortality — 6 to 11 deaths per 100,000 people — constitute a significant burden.

Influenza epidemics have been most extensively studied in temperate regions like Western Europe, where they occur during the winter months. While influenza cases tend to peak during the rainy season in tropical and subtropical regions like Bangladesh and Vietnam, long-term trends and within-region variation in seasonality have been relatively understudied in these regions. In addition, it remains unclear to what extent influenza epidemics in poultry — avian influenza — coincide with or drive epidemics in human populations. icddr,b scientists were part of a recently published study in The Lancet Global Health that addressed these questions in Bangladesh.

The authors used data on influenza cases between 2010 and 2019 from 32 hospitals across Bangladesh to investigate when epidemics peaked and whether there was variation in intensity and timing in different parts of the country. To estimate avian influenza seasonality, the authors used live bird market surveillance data from the capital city of Dhaka collected between 2016 and 2019.


Monthly time series of influenza cases and samples tested from hospital-based sentinel surveillance in Bangladesh, 2010–19. Bars indicate the monthly number of positive influenza cases. The dashed line indicates the monthly number of samples tested.

While human influenza epidemics tended to occur during the rainy season as observed in previous studies, the authors discovered significant regional variation in the timing of epidemic peaks. Strikingly, influenza cases peak earliest — around mid-June — in the center of the country in the capital city of Dhaka and then radiate outward through the surrounding districts (Figure 1). This has major implications for predicting the course of seasonal flu in the country and taking preventative public health measures to limit its spread and impact.

Avian influenza showed very weak seasonality — with a small peak during April — and occurred throughout the year without correlation with human influenza epidemics. Circulation of avian influenza throughout the year worryingly suggests that there is constant risk of zoonotic transmission into human populations. While avian influenza viruses are distinct from human influenza and preferably infect birds, they can mutate and jump between hosts with the risk of emergence of novel influenza strains with pandemic potential in the region.

Flu vaccines which save thousands of lives every year when widely administered — are still not used nor promoted at the policy level in Bangladesh, but that must change given the virus’s disease burden in the country. Considering the change in circulating seasonal influenza virus strains, every year, a new influenza vaccine is developed and administered. The results of studies like this one can help guide the timing of national vaccination programs in the future. Temperate countries tend to begin vaccinations in autumn, but this study indicates that the timing would need to be in the summer or late spring in Bangladesh.

Vaccines are not the only available intervention against influenza. Decreased incidence of influenza in many countries as a byproduct of interventions against SARS-CoV-2 has shown that influenza virus epidemics can be restricted in the future through the adoption of behaviors such as masking and quarantine when cases are emerging. Knowing the temporal and geographical pattern of spread of the virus can help direct efforts to control transmission and massively reduce the burden of influenza in the country.

“Influenza has now come under the shadow of SARS-CoV-2 because of the COVID-19 pandemic,” said Dr. Fahmida Chowdhury, Associate Scientist at icddr,b and a co-author of the study. “But you have to remember that this is the virus that caused the swine flu pandemic as recently as in 2009 and remains a constant pandemic threat that we must continue to surveil in human and bird populations. On top of that, seasonal influenza claims thousands of lives every year, and these deaths are preventable. We are hopeful that our results can guide interventions aimed at prevention and control of future influenza epidemics.”


Ornob Alam