Pandemic (H1N1) 2009 among Quarantined Close Contacts, Beijing, People’s Republic of China

Xinghuo Pang¹, Peng Yang¹, Shuang Li, Li Zhang, Lili Tian, Yang Li, Bo Liu, Yi Zhang, Baiwei Liu, Ruogang Huang, Xinyu Li, and Quanyi Wang

Author affiliations: Beijing Center for Disease Prevention and Control, Beijing, People’s Republic of China; Capital Medical University School of Public Health and Family Medicine, Beijing

Abstract

We estimated the attack rate of pandemic (H1N1) 2009 and assessed risk factors for infection among close contacts quarantined in Beijing, People’s Republic of China. The first 613 confirmed cases detected between May 16 and September 15, 2009, were investigated; 7,099 close contacts were located and quarantined.

The attack rate of confirmed infection in close contacts was 2.4% overall, ranging from 0.9% among aircraft passengers to >5% among household members. Risk factors for infection among close contacts were younger age, being a household member of an index case-patient, exposure during the index case-patient’s symptomatic phase, and longer exposure. Among close contacts with positive test results at the start of quarantine, 17.2% had subclinical infection. Having contact with a household member and younger age were the major risk factors for acquiring pandemic (H1N1) 2009 influenza virus infection. One person in 6 with confirmed pandemic (H1N1) 2009 was asymptomatic.

In early April 2009, human cases of infection with a novel influenza virus of swine origin, pandemic (H1N1) 2009 virus, were identified in the United States and Mexico, and this virus spread rapidly across the world (1–3). On June 11, 2009, the World Health Organization raised the pandemic level to 6, the highest level for pandemic alert (4).

Estimating attack rates is a major task in characterizing pandemic (H1N1) 2009. Some studies have reported attack rates of pandemic (H1N1) 2009 among household members and aircraft passengers (5–7). These studies suggested that the transmissibility of pandemic (H1N1) 2009 virus was low. These studies were conducted in outbreak settings, and attack rates were calculated on the basis of clinical diseases that included influenza-like illness (ILI) or acute respiratory illness (ARI) of close contacts rather than confirmed infection with pandemic (H1N1) 2009 virus. In addition, in these studies only symptomatic index and secondary cases were included. Although most infections of pandemic (H1N1) 2009 influenza virus produce ILI or ARI symptoms (8–12), subclinical infection can occur and can change the estimate of attack rate. In addition, the infectivity of asymptomatic case-patients has not been clearly defined (13).

Because of the high rates of illness and death among the initial case-patients with pandemic (H1N1) 2009 (14), the Chinese government decided to prevent and contain the rapid spread of disease through tracing and quarantine of persons who had close contact with persons with confirmed cases of pandemic (H1N1) 2009. Beijing, the capital of the People’s Republic of China, took strict containment and control measures through October 2009. The Beijing municipal government implemented border entry screening, ILI screening in hospitals, health follow-up of travelers from overseas, and quarantine and testing of close contacts to identify new introduction of cases and local transmission. Public health workers conducted epidemiologic investigation of all index case-patients (including those with subclinical infections) and traced and quarantined close contacts whose residence was within the jurisdiction of Beijing. We estimated the attack rate of pandemic (H1N1) 2009 virus infection and assessed risk factors or correlates for infection among different types of close contacts, including household members and aircraft passengers.

Methods

Confirmation of Index Cases

full-text:
Pandemic (H1N1) 2009 among Quarantined Close Contacts, Beijing, People’s Republic of China - Vol. 17 No. 10 - October 2011 - Emerging Infectious Disease journal - CDC

Suggested citation for this article: Pang X, Yang P, Li S, Zhang L, Tian L, Li Y, et al. Pandemic (H1N1) 2009 among quarantined close contacts, Beijing, People’s Republic of China. Emerg Infect Dis [serial on the internet]. 2011 Oct [date cited]. http://dx.doi.org/10.3201/eid1710.101344