Non-annual seasonality of influenza-like illness in a tropical urban setting

Abstract

In temperate countries, influenza and other viral respiratory diseases often have distinct seasonal peaks occurring during colder, wintertime months. However, little is known about the dynamics of influenza and viral respiratory disease dynamics in the tropics, despite high morbidity and a clear epidemiological link between tropical and temperate countries. In temperate countries, the dynamics of influenza and other respiratory diseases are often analyzed using syndromic surveillance data describing influenza-like illness (ILI) as ILI is highly correlated with virological surveillance for influenza. To obtain a detailed picture of respiratory disease incidence patterns in a large tropical city, we established an mHealth study in community outpatient clinics in Ho Chi Minh City, Vietnam (11N latitude). From August 2009 through December 2015, clinics reported daily case numbers of ILI using standard mobile-phone SMS messaging. A subset of these clinics performed molecular diagnostics for influenza A and B viruses. Unlike the annual patterns seen in temperate countries, ILI activity in Ho Chi Minh City exhibited strong non-annual periodicity and was not correlated with PCR-confirmed influenza. The dominant periodicity in the data was approximately 200 days. This was confirmed by a time series decomposition, a step-wise regression analysis on annual and non-annual covariates, and a forecasting exercise showing that forecasting was 30% to 40% more accurate when a 200-day non-annual cycle was included in the forecast. This suggests, for the first-time, that a non-annual cycle may be an essential driver of ILI dynamics in the tropics. This raises new questions about the seasonality and drivers of respiratory disease transmission in tropical countries.

Primary care influenza‐like illness surveillance in Ho Chi Minh City, Vietnam 2013‐2015

“…The ability of current ILI definitions to discriminate between influenza and non‐influenza causes of upper respiratory tract infection in tropical settings has previously been called into question . The clinics in this study also participated in a trial of mobile phone reporting of ILI which demonstrated that ILI symptoms are much more consistent through the year compared to temperate countries and that peaks in ILI activity do not always correlate with peaks in influenza activity . The results of our study demonstrated that although fever, cough and rhinorrhoea were reported more commonly in those who had influenza, such symptoms also occurred in more than 70% of patients with non‐influenza ILI confirming their poor discriminating ability.…”

“…This observational study was conducted between August 2013 and November 2015 at outpatient clinics and community medical practitioners in Ho Chi Minh City (HCMC), Vietnam (private clinics were drawn from the an ongoing mHealth surveillance study ). These clinics were considered representative of primary care within Vietnam .…”

“…Year‐round transmission of influenza has been detected in Vietnam through both national surveillance and phylogenetic analysis . Simultaneous circulation of multiple influenza types and subtypes has been documented along with patients having more than one influenza infection in one season . Approximately 15%‐20% of patients presenting to hospitals with influenza‐like illness (ILI) have virologically confirmed influenza, and influenza is associated with up to 14% of community‐acquired pneumonia presenting to hospitals .…”