Incidence of pandemic (H1N1) 2009 influenza infection in children and pregnant women during the 2009 influenza season in Western Australia — a seroprevalence study

Abstract

Objective: To determine antibody levels and estimate incidence of infection with pandemic (H1N1) 2009 influenza in children and pregnant women during the 2009 winter in Western Australia. Design, setting and participants: Two cross‐sectional serosurveys using stored specimens collected for unrelated pathology testing, from before and after (3 August to 30 November 2009) circulation of the pandemic virus, and before commencement of the pandemic vaccination program. Specimens were from three groups: children aged 1–4 years, older children and teenagers aged 5–19 years, and pregnant women aged 21–45 years. The groups were geographically representative of the WA population. Main outcome measures: Reactivity against pandemic (H1N1) 2009 and seasonal A(H1N1) influenza viruses measured using haemagglutination inhibition (HI) assays. Results: Antibody titres were determined for 648 individuals in the prepandemic period and 736 in the postpandemic period. In the prepandemic period, HI titres ≥ 40 against the pandemic virus were found in 0 (95% CI, 0.0%–1.6%) children aged 1–4 years, 8.3% (95% CI, 5.3%–12.7%) of older children and teenagers, and 4.5% (95% CI, 2.4%–8.3%) of pregnant women. In postpandemic specimens collected from 1 September 2009 (when influenza activity had declined to near‐baseline levels), estimated infection rates (subtracting prepandemic levels) were 25.4% (95% CI for difference, 18.6%–33.4%) in 1–4‐year‐old children, 39.4% (95% CI, 29.8%–48.5%) in older children and teenagers, and 10.2% (95% CI, 4.1%–17.1%) in pregnant women. Conclusions: A quarter of preschool children and about 40% of school‐aged children and older teenagers had serological evidence of pandemic influenza infection during winter 2009, indicating high levels of mild or asymptomatic infection. The infection rate in pregnant women was much lower. The high infection rates in children help explain the reduced impact of the pandemic virus during the 2010 winter. Augmented by vaccination, there should be sufficiently high levels of immunity in the Australian population to significantly reduce the impact of the virus in future influenza seasons.

Prevalence of 2009 Pandemic Influenza A (H1N1) Virus Antibodies, Tampa Bay Florida — November–December, 2009

“…Our seroprevalence estimates are similar to previously published studies from Pittsburgh, PA [10] and international studies that collected sera at comparable time periods [11], [13], [15], [16], [20], [24], [36]. Consistent with other studies, school-aged children were estimated to have had the highest seroprevalance of pH1N1 antibodies [10], [11], [12], [13], [15], [24].…”

“…Additional studies performed throughout the world have also been published, adding to the body of literature describing the disease burden of the pH1N1 pandemic [9], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24]. The objective of our study was to determine the seroprevalence of pH1N1 antibodies among residents of Tampa Bay, Florida after the peak of the fall wave and prior to widespread vaccination.…”

Epidemiological Characteristics of 2009 (H1N1) Pandemic Influenza Based on Paired Sera from a Longitudinal Community Cohort Study

“…Our results can be compared with serology-based studies of influenza incidence in other populations during the 2009 (H1N1) pandemic [4]–[16]. In England and Wales, a study of cross-sectional clinical samples found substantial increases in the proportion of younger children with titres 1∶32 or greater between a 2008 baseline ( n =  1,403) and sample taken in September 2009 ( n =  1,954), thus giving valuable early evidence that the infection attack rate was high in some age groups and, hence, that the rate of severe cases per infection in the most affected age groups was likely to be low [10].…”

“…Previous community-based serological surveys of populations outside Hong Kong have established a broad consistent pattern for the 2009 influenza pandemic, namely, high rates of infection in school-aged children relative to younger adults and lower rates in older adults: Australia [4]–[7]; Belgium [8]; China [9]; Costa Rica [8]; England and Wales [10]; Germany [8]; India [11]; Japan [8]; New Zealand [12]; Scotland [13]; Singapore [14]; Thailand [15]; and the United States of America [8],[16]. Also, our own previous work has established a similar age-specific pattern of infection during the first wave in Hong Kong and per-infection mortality rates that escalated sharply with age, [17].…”

Epidemiological aspects of influenza A related to climatic conditions during and after a pandemic period in the city of Salvador, northeastern Brazil

“…In Perth, Australia, a quarter of preschool children and approximately 40% of school-aged children and older teenagers had serological evidence of pandemic influenza infection during the winter of 2009; the authors suggest that high levels of mild or asymptomatic infection must have occurred in the population (Dowse et al 2011). The clinical syndrome caused by A(H1N1)pdm09 ranged from febrile illness to severe pneumonia; however, the majority of clinical presentations were a influenza-like illness with fever and cough that was occasionally accompanied by dyspnoea, sore throat and runny nose (WC-WHO 2010)…”