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

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

doi:10.3201/eid1710.101344

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.

Highlights

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
For symptomatic close contacts infected with pandemic (H1N1) 2009, the median of generation time were 2.4 days; it was shorter for flight passenger contacts than nonpassenger contacts (1.6 days vs. 2.5 days) (Table 1)
We estimated that pandemic (H1N1) 2009 virus was transmitted by 18% of index case-patients to their close contacts and that 2.4% (167 of 7,099) of close contacts we traced were infected

Summary

Introduction

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

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