Influenza Seasonality in the Tropics and Subtropics – When to Vaccinate?

Abstract

BackgroundThe timing of the biannual WHO influenza vaccine composition selection and production cycle has been historically directed to the influenza seasonality patterns in the temperate regions of the northern and southern hemispheres. Influenza activity, however, is poorly understood in the tropics with multiple peaks and identifiable year-round activity. The evidence-base needed to take informed decisions on vaccination timing and vaccine formulation is often lacking for the tropics and subtropics. This paper aims to assess influenza seasonality in the tropics and subtropics. It explores geographical grouping of countries into vaccination zones based on optimal timing of influenza vaccination.MethodsInfluenza seasonality was assessed by different analytic approaches (weekly proportion of positive cases, time series analysis, etc.) using FluNet and national surveillance data. In case of discordance in the seasonality assessment, consensus was built through discussions with in-country experts. Countries with similar onset periods of their primary influenza season were grouped into geographical zones.ResultsThe number and period of peak activity was ascertained for 70 of the 138 countries in the tropics and subtropics. Thirty-seven countries had one and seventeen countries had two distinct peaks. Countries near the equator had secondary peaks or even identifiable year-round activity. The main influenza season in most of South America and Asia started between April and June. The start of the main season varied widely in Africa (October and December in northern Africa, April and June in Southern Africa and a mixed pattern in tropical Africa). Eight “influenza vaccination zones” (two each in America and Asia, and four in Africa and Middle East) were defined with recommendations for vaccination timing and vaccine formulation. The main limitation of our study is that FluNet and national surveillance data may lack the granularity to detect sub-national variability in seasonality patterns.ConclusionDistinct influenza seasonality patterns, though complex, could be ascertained for most countries in the tropics and subtropics using national surveillance data. It may be possible to group countries into zones based on similar recommendations for vaccine timing and formulation.

Establishing seasonal and alert influenza thresholds in Cambodia using the WHO method: implications for effective utilization of influenza surveillance in the tropics and subtropics

“…We consider channels such as press releases, the Internet, posters and the National Respiratory Disease and Influenza Bulletin to convey these messages. In the long-term, the seasonal threshold will be helpful for vaccination timing; ( 10 , 11 , 21 ) continuous re-evaluation of vaccination timing will be necessary as the timing of season onset has been observed to vary in the tropics.…”

“…( 14 - 16 , 19 ) Lack of apparent ILI seasonality could be unique in the tropics and subtropics with various pathogens circulating year-round that cause acute respiratory illnesses. ( 8 - 11 , 21 - 32 ) Instead, use of proportion positive and composite approaches may be suggested given recent studies with laboratory information indicating that most countries, including non-temperate countries, exhibit distinct seasonal patterns. ( 8 - 11 , 21 - 32 ) Our findings regarding the usefulness of the composite variable agree with those from a temperate region in Australia ( 19 ) and highlight the importance of using multiple sources of information to guide assessment.…”

“…( 3 - 7 ) Furthermore, while yearly variations may occur, seasonality also appears to exist in most tropical and subtropical regions. ( 8 - 11 ) Therefore, it is essential to analyse influenza surveillance data in a practical and efficient manner to inform decision-making regarding influenza in the tropics and subtropics.…”

Progress and Remaining Gaps in Estimating the Global Disease Burden of Influenza

“…The growth of influenza surveillance and research in the past 10 years has generated substantial new data on the epidemiology and risk from influenza around the world, notably in tropical countries and LMICs ( 13 – 16 ). These data have confirmed that influenza is a major cause of hospitalization and severe acute respiratory disease in all settings, whether rich or poor, tropical or temperate, urban or rural ( 17 – 19 ), and that the risk for severe influenza outcomes might be higher in LMICs than in high-income countries ( 20 ).…”

Reply to Alonso et al. “Bangladesh and Rwanda: Cases of high burden of influenza in tropical countries?”

“…However, we are not convinced that this comparison is generalizable to all tropical countries. For instance, not all tropical countries have year‐round influenza virus circulation as the authors suggest, including Bangladesh and Rwanda 14. Bangladesh has a distinct peak in influenza virus circulation during the monsoon season from April to September each year,15 whereas Rwanda experiences increased influenza virus circulation during the long rainy season from January to July 16.…”