Serial intervals and the temporal distribution of secondary infections within households of 2009 pandemic influenza A (H1N1): implications for influenza control recommendations

Abstract

A critical issue during the 2009 influenza A (H1N1) pandemic was determining the appropriate duration of time individuals with influenza-like illness (ILI) should remain isolated to reduce onward transmission while limiting societal disruption. Ideally this is based on knowledge of the relative infectiousness of ill individuals at each point during the course of the infection. Data on 261 clinically apparent pH1N1 infector-infectee pairs in households, from 7 epidemiological studies conducted in the United States early in 2009, were analyzed to estimate the distribution of times from symptom onset in an infector to symptom onset in the household contacts they infect (mean, 2.9 days, not correcting for tertiary transmission). Only 5% of transmission events were estimated to take place >3 days after the onset of clinical symptoms among those ill with pH1N1 virus. These results will inform future recommendations on duration of isolation of individuals with ILI.

Figure 1.

The distribution of the clinically apparent serial interval by study based on acute respiratory illness (ARI) (a) and on influenza-like illness (ILI) (b), and the combined distribution of the ARI clinically apparent serial interval based on all studies (c).

Figure 2.

The maximum likelihood Weibull distributions for the true serial interval conditional on it being between 1 and D_max inclusive (a) and the corresponding unconditional distributions (b): for the primary analysis (based on acute respiratory illness [ARI] assuming D_max = 7 days) and accompanying analyses (based on ARI assuming D_max = 10 days and based on influenza-like illness [ILI] assuming D_max = 7 days).

Figure 3.

The estimated distribution of transmission events, relative to the onset of clinical symptoms on the index case obtained from the deconvolution of the maximum likelihood Weibull serial interval distribution: for the primary analysis (based on acute respiratory illness [ARI] assuming D_max = 7 days), and accompanying analyses (based on ARI assuming D_max = 10 days and based on influenza-like illness [ILI] assuming D_max = 7 days).