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There are ideal seasons for vaccinating against influenza and these are different in the temperate and tropical regions of the world, according to a new international study.

In temperate areas the viral disease peaks during the winter months — November to February in the northern hemisphere, and May to October in the southern hemisphere, says the study published April in PLOS ONE.

Most tropical countries, in contrast, experience multiple peaks of influenza activity each year, with disease incidence peaking not just during winters but also during the rainy season, according to the study by Siddhivinayak Hirve of the global influenza programme of the WHO and an international team of researchers.

After studying laboratory-confirmed influenza activity in about 70 countries in the tropics and subtropics, together housing 73 per cent of the world’s population, the team could discern distinct patterns of disease onset in 41 countries.

“In countries like Kenya, Madagascar and Malaysia, we could not identify any patterns. There was near-continuous influenza activity,” says Hirve. Such countries, and those that traverse many latitudes, like India, may have to refine their vaccination strategies based on local patterns, the study recommends. In India, influenza incidence coincides with the monsoon rains across most of the country, but there is a secondary peak in the Himalayas during the winter months, says Hirve. “Following a staggered approach to vaccination presents logistical challenges. Choosing different vaccine formulations for different parts of the country is also a challenge,” Hirve tells SciDev.Net.  

“Influenza viruses have been shown to survive better in low humidity and low temperature settings and, therefore, they survive longer and spread better in winter than in summer,” says Nancy Cox, a virologist and former leader of influenza research at the Centres for Disease Control, Atlanta.  

About 10 per cent of the world’s population gets affected by influenza annually. Each year, WHO studies surveillance data from 120 countries across the world and formulates vaccines accordingly.

Grouping countries together, the authors of the study identify ‘influenza vaccination zones’ — two for America and Asia, one for North Africa and Middle East and separate zones for Saharan, equatorial and southern Africa.

“They have used multiple data sources for comparison so as to reach the best conclusions,” commented Cox.

This piece was produced by SciDev.Net’s South Asia desk.

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