Symptomatic hMPV
From 2011–2016, of the 2576 children that participated in the cohort 478 (18.6%) had at least one RT-PCR confirmed hMPV illness episode (Table 1). The overall symptomatic incidence rate was 5.74 (95% CI 5.3, 6.2) per 100 person-years (Supplemental Table 2). Most symptomatic hMPV illness occurred in the first year of life (Fig. 2), with the highest incidence rate occurring between 6–8 months (23.5 cases per 100 person years 95% CI 16.8, 32.0). The lowest incidence was observed in the 10–12 age group (1.0 case per 100 person years 95% CI 0.6, 1.7). Children under one year old had 3.2 times the incidence of children one year or older (Rate Ratio 3.2 95% CI 2.6, 4.0 p < 0.001). There was no significant difference (p > 0.05) observed between sexes (Supplemental Table 2).
Of the total 564 hMPV symptomatic cases, 87 (15%) were symptomatic reinfections (Table 1, Fig. 3). The median time to reinfection was 22 months, or 1.8 years (min 1 month, max 5.1 years), and the median age of reinfection was 40 months, or 3.3 years (min 6 months, max 10.2 years). Participant age was significantly associated with the amount of time between positive RT-PCRs (p < 0.001). Most secondary symptomatic infections occurred later in life (4–14 years), compared to the first 3 years of life (Fig. 3B).