The early arrival of the influenza season overlapping usual hRSV season, the circulation of a drifted influenza virus not covered by vaccine and the high prevalence of risk factors for infection and severity in the village jointly can explain the high attack rate of ARI, even with a high rate of influenza vaccination.
Test-positive subjects were generally older, more likely to present with one of the symptoms of ARI, and less likely to receive vaccination against influenza.
An influenza virus was identified in an estimated 78% (95% CI: 75, 81) and 17% (95% CI: 15, 21) of respiratory hospitalizations attributed to influenza for children and adults, respectively, and 75% of influenza-attributed hospitalizations had an ARI diagnosis.
Our results indicate that (a) VE estimates may suffer from substantial confounding bias when a confounder has a different effect on the probabilities of influenza and non-influenzaARI, and (b) when vaccination reduces the probability of seeking care against influenzaARI, then estimates of VE against MAI may be unbiased while estimates of VE against SI may be have a substantial positive bias.
Sensitivity(95% CI) and specificity (95% CI) for influenza infection were 78% (67-87) and 60% (57-63) for ILI (measured/reported fever); 37% (26-49) and 78% (75-80) for SARI (measured/reported fever); 82% (72-90) and 57% (54-60) for FARI (measured/reported fever); 88% (78-94) and 45% (42-49) for ARI; and 74% (63-84) and 61% (58-64) for measured/reported fever plus cough.