Transmission of Respiratory Syncytial Virus Among Children Under 5 Years in Households of Rural Communities, the Philippines

Otomaru, Hirono; Kamigaki, Taro; Tamaki, Raita; Okamoto, Michiko; Alday, Portia Parian; Tan, Alvin Gue; Manalo, Joanna Ina; Segubre-Mercado, Edelwisa; Inobaya, Marianette Tawat; Tallo, Veronica; Lupisan, Socorro; Oshitani, Hitoshi

In the 181 households included in the analysis, 396 children were <5 years old, including 212 children with RSV-ARI and 184 children without RSV-ARI infection. In addition, 152 of the 181 households (84.0%) had 1 RSV-ARI case each (RSV-HH1) and 29 (16.0%) had two or more RSV-ARI case each (RSV-HH2), including 2 households (1.1%) with 3 RSV-ARI cases each. Thus, a total of 212 RSV-ARI were identified, which included 181 RSV-ARI as a primary case (152 single RSV-ARI and 29 1st RSV-ARI), and 31 2nd RSV-ARI of which onset dates were after the 1st RSV-ARI. However, 2 2nd RSV-ARI cases had the same date of onset. Supplementary Table 1 shows the RSV-ARI cases found in RSV-HH2. We found only 5 2nd RSV-ARI episodes in young infants (<6 months old) (see Supplementary Table 1) . Although, the infection source of 31 young infants identified as a single RSV-ARI in RSV-HH1 remained unclear ( Table 1) . One of the objectives of this study was to define the infection source, especially in young infants. Because we conducted a household visit biweekly, we could not collect a specimen from all of ARI episodes. It was possible that other children <5 years old with ARI were a potential source of RSV infection for young infants. Therefore, we further analyzed ARI cases without samples from children <5 years old of RSV-HH1, including preceding ARI or subsequent ARI cases, by assuming that these ARI cases were probably due to RSV. (Table 3) . A summary of total preceding cases and total subsequent ARI cases is shown in Supplementary Figure 3 . We calculated the interval between the dates of onset of 1st RSV-ARI and 2nd RSV-ARI for 31 pairs of children with RSV-ARI in 29 RSV-HH2 households. We did not include RSV-HH1 in this analysis because only 1 case in the household was confirmed as RSV-ARI. The highest interval frequencies were observed on days 1 and 2 (n = 7; 22.6% each) (see Supplementary Figure 4 ). The estimated mean serial interval was 3.2 days (95% CI, 2.5-3.8), 3.2 days (95% CI, 2.5-3.9), and 3.2 days (95% CI, 2.5-4.1) in the gamma, Weibull, and log-normal distributions, respectively. Supplementary Figure 4 shows the cumulative proportions fitted by parametric models. The gamma model generated a slightly favorable AIC.