Influenza season has changed

It was once widely believed that winter is the season for influenza, a contagious viral infection of the respiratory system. However, epidemiologists now report a significant shift in this pattern. Influenza is currently appearing most frequently during the summer and monsoon seasons. Alongside seasonal influenza, several cases of avian influenza or bird flu have also been detected among humans this year. The public health experts are viewing it as a new potential risk.

Discussing this shift in influenza trends, Professor Tahmina Shirin, Director of the Institute of Epidemiology, Disease Control and Research (IEDCR) told Prothom Alo, “Our regular surveillance indicates that influenza remains more prevalent for about six months. It begins to rise in April and starts to decline from September. The highest number of cases is observed in June, July, and August, during the summer season, while winter shows the lowest incidence.”

Public health experts identify five groups as being at the highest risk of influenza infection: the elderly, children less than five years of age, individuals with comorbidities, pregnant women and healthcare workers.

Since 2007, the IEDCR and the International Centre for Diarrhoeal Disease Research, Bangladesh (icddr,b) have jointly conducted influenza surveillance in 19 government hospitals -  nine district hospitals and 10 tertiary-level hospitals. To monitor the situation, patient samples are regularly tested, which also helps detect other infections, including bird flu.

An analysis of samples collected between January 2007 and December 2024 revealed that July consistently recorded the highest proportion of influenza-positive cases, with an average detection rate of 30.85 per cent. The data from the past 17 years show that detection rates begin increasing from April (11.86 per cent), continue to rise through May (21.86 per cent) and June (29.10 per cent), peak in July and then decline through August (22.25 per cent) and September (18.86 per cent).

The lowest detection rates occur during December (2 per cent) and January (1.74 per cent).

A review of data from the 19 hospitals for the current year indicates that influenza was detected in 2 per cent of samples in January, 3 per cent in February, and 4 per cent in March. The rate increased to 7 per cent in April, 12 per cent in May and 29 per cent in June. The highest rate was recorded in July (67 per cent), followed by August (62 per cent) and it declined again in September (16 per cent). IEDCR officials anticipate that detection rates will continue to decrease in October.

During testing of hundreds of samples across the 19 hospitals, four samples were found to contain the avian influenza (H5N1) virus. The first human case of bird flu this year was identified in February, followed by two cases in April and May and another in August.

Officials believe that poultry farms and live bird markets are the primary sources of infection. It is assumed that all four affected individuals were exposed either through farms or markets. This suggests that the avian influenza virus is present alongside the seasonal influenza virus.

Professor Tahmina Shirin commented, “The simultaneous presence of two viruses poses a significant risk. Mutations or genetic recombinations between the two strains could potentially lead to the emergence of a new virus. We cannot say that we are completely free from that risk, and we must remain vigilant.”

Public health experts advise that individuals with cold or flu-like symptoms should self-isolate. They should wear masks, wash hands frequently, follow cough and sneeze etiquette and avoid crowded places.

According to public health specialist MHM Alamgir, “In western countries, influenza peaks during winter, but in Bangladesh, the season extends across the summer and monsoon months. Adhering to proper health practices can help prevent infection. A vaccine is also available. Taking the influenza vaccine between January and February provides protection for several months, though it must be administered annually.”