Influenza A(H1N1)pdm09 outbreak detected in inter-seasonal months during the surveillance of influenza-like illness in Pune, India, 2012-2015

Abstract

An outbreak of influenza A(H1N1)pdm09 was detected during the ongoing community-based surveillance of influenza-like illness (ILI). Among reported 119 influenza A(H1N1)pdm09 cases (59 cases in the year 2012 and 60 cases in 2015) in summer months, common clinical features were fever (100%), cough (90·7%), sore throat (85·7%), nasal discharge (48·7%), headache (55·5%), fatigue (18·5%), breathlessness (3·4%), and ear discharge (1·7%). Rise in ILI cases were negatively correlated with the seasonal factors such as relative humidity (Karl Pearson's correlation coefficient, i.e. r = -0·71 in the year 2012 and r = -0·44 in the year 2015), while rise in ILI cases were positively correlated with the temperature difference (r = 0·44 in the year 2012 and r = 0·77 in the year 2015). The effective reproduction number R, was estimated to be 1·30 in 2012 and 1·64 in 2015. The study highlights the rise in unusual influenza activity in summer month with high attack rate of ILI among children aged ⩽9 years. Children in this age group may need special attention for influenza vaccination. Influenza A(H1N1)pdm09 outbreak was confirmed in inter-seasonal months during the surveillance of ILI in Pune, India, 2012-2015.

Keywords: Community surveillance; disease outbreak; influenza A(H1N1)pdm09; seroepidemiology; transmission dynamics; urban slum.

Fig. 1.

Study area and study population. (a) Location of community site in Pune city, Maharashtra, India. (b) Population pyramid showing age and sex distribution of study population.

Fig. 2.

(a) Time distribution of incidence density of influenza-like illness cases showing influenza etiology (b) sampling proportion from year January 2012 to April 2015. (c) Time distribution of influenza like illness cases showing influenza etiology and its correlation with seasonal factors during influenza A(H1N1)pdm09 outbreak in year 2012 and (d) in year 2015.

Fig. 3.

Age-specific attack rate of influenza-like illness cases per 1000 persons showing influenza etiology in (a) January–April 2012 and (b) January–April 2015.

Fig. 4.

HA gene phylogenetic analysis of 2012 and 2015 influenza A(H1N1)pdm09 strains from the study denoted with solid black triangle and compare with viruses from severe cases of same period. The vaccine component is shown in red color.

Fig. 5.

Cumulative growth of confirmed cases of influenza A(H1N1)pdm09 with time. Data plot and curve fitting in MATLAB for (a) year 2012 (data set: 21 February–22 March 2012) and (b) year 2015 (data set: 1 February–21 March 2015).