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CDC Collaborative Study: Influenza Seasonality in the Tropics and Sub-tropics

A new collaborative study by CDC and global health partners published today in the journal PLOS ONE examined the timing of influenza virus circulation in 138 tropical and sub-tropical countries and territories and provided new evidence which may help inform the choice of influenza vaccine formulation and the timing of vaccination campaigns.

Historically, the timing of the bi-annual World Health Organization (WHO) influenza vaccine composition selection and production cycle has been directed by the influenza seasonality patterns in the temperate regions of the northern and southern hemispheres. Influenza activity in the tropics and sub-tropics has been less well defined because these countries often identify influenza throughout the year and some have multiple peaks of influenza activity each year. However, in recent years, influenza surveillance has improved in many countries, providing the opportunity for researchers to better define influenza seasonality. The study results have significant implications for the increasing number of low- and middle-income countries in the tropics and sub-tropics which over the last decade have begun introducing seasonal influenza vaccination into their immunization programs or are expanding their programs to include maternal influenza immunization, especially in the Latin American region.

Periods of peak flu activity were identified for 70 of the 138 countries examined, representing about 73% of the world’s population. Thirty-seven countries had one distinct influenza peak and 17 countries had two distinct peaks each year. Countries near the equator often identify influenza throughout the year and had secondary peaks. Researchers determined that most countries in Central and South America, South and Southeast Asia experienced a primary period of influenza activity from April to June. India showed an additional secondary peak between October and December. Africa presented a complex picture with increased activity from October to December in the northern region, from April to June in the southern region and throughout the year in sub-Saharan countries near the equator.

These findings suggest that optimal timing for an annual seasonal influenza vaccination campaign could be identified for most countries in the tropics and sub-tropics. A southern hemisphere formulation is recommended to be given in April for most of Central and South America (with the exception of Guatemala, Jamaica and Mexico), sub-Saharan Africa (with the exception of the Democratic Republic of Congo, Malawi, Rwanda and the United Republic of Tanzania), and tropical Asia (with the exception of Sri Lanka and Indonesia). A northern hemisphere formulation is recommended to be given in October for northern Africa and the Middle East up to Pakistan. Countries such as Brazil, China and India which have varied flu seasonality and countries near the equator with year-round influenza activity could explore the marginal benefit of alternate vaccination times based on local flu seasonality.

The study was conducted using four different statistical approaches by researchers from CDC, Netherlands Institute for Health Services Research (NIVEL), Program for Appropriate Technology (PATH) and the WHO. Researchers analyzed laboratory-confirmed influenza activity data for the seasons 2010 to 2015 from the WHO’s FluNet reporting system, the largest global database for influenza virological surveillance data. National surveillance data for the seasons 2002 to 2014 were also used, where available. Influenza pandemic years 2009-2010 were excluded, as this time period was not seasonal by definition.

While the study has proposed simplified operational guidance to countries in the tropics and sub-tropics, researchers emphasize that once the timing of vaccination is determined by local seasonality, the most recent WHO influenza virus vaccine recommendation should be used, regardless of the geographic location of the country.

This study is available online in PLOS ONE.

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