Interpreting the transmissibility of measles in two different post periods of supplementary immunization activities in Hubei, China Ka Chun Chong a,b,1 , Chi Zhang c,1 , Benny Chung Ying Zee a,b , Tongyong Luo a , Lei Wang c , Greta Chun Huen Tam a , Katherine Min Jia a , Riyang Sun a,b , Maggie Haitian Wang a,b,⇑ , Xuhua Guan c,⇑ a JC School of Public Health and Primary Care, The Chinese University of Hong Kong, Hong Kong, China b Clinical Trials and Biostatistics Laboratory, Shenzhen Research Institute, The Chinese University of Hong Kong, China c Hubei Provincial Center for Disease Control and Prevention, Hubei, China article info Article history: Received 7 October 2016 Received in revised form 3 January 2017 Accepted 5 January 2017 Available online 19 January 2017 Keywords: Measles Reproduction number Next generation matrix Supplementary immunization Vaccination abstract Although evidence has shown that supplementary immunization activity (SIA) campaigns greatly reduce the incidence of measles, their effects on disease transmissibility have seldom been monitored. A great decrease in the number of cases may be a false signal of early success towards measles elimination to policy makers. By interpreting the transmissibility in two different post-SIA periods in Hubei, China, the current study showed sustained measles transmissions despite a reduced number of cases. Two population-based cross-sectional serological surveys of measles antibodies were conducted in Hubei pro- vince in mid-2010 and mid-2011 after the implementation of SIAs. Immunoglobulin G (IgG) antibodies against measles were measured by enzyme-linked immunosorbent assay (ELISA). Based on the estimated age-specific susceptibility levels, the effective reproduction number (R), a key indicator of disease trans- missibility, was determined by the next generation matrix in transmission model. The results revealed an overall IgG seroprevalence of 88.0% (95% confidence interval [CI]: 85.6–90.4%) and 89.6% (95%CI: 88.0– 91.2%), respectively, in the two different periods. Comparatively lower seroprevalence rates were observed among children less than 24 months of age and young adults 15 to 19 years of age in 2011. The Rs were 0.76 and 1.53 for the two study periods. In conclusion, even though the incidence was reduced to below 1/100,000 in both 2010 and 2011, the reproduction number in 2011 indicates a high risk for sustained measles transmission. This finding was potentially due to a lower seropositivity rate among young adults that had not been covered in the first SIA. Thus, implementation of SIA targeted to appropriate age groups is recommended. Regular monitoring of seroprevalence is also suggested to track disease transmissibility and to align SIA with the appropriate age groups. Ó 2017 Elsevier Ltd. All rights reserved. 1. Introduction Measles, an infectious disease caused by the highly contagious measles viruses, contributes to childhood morbidity and mortality worldwide. Before vaccinations against measles were developed, 90% of children were infected with the virus before 15 years of age, corresponding to millions of lives worldwide lost to measles-induced diseases each year [1]. Since the initiation of large-scale vaccination programs from the 1920s to the 1960s, the incidence rate of measles has declined sharply. However, measles still tops the list of fatal infectious diseases worldwide, especially in developing countries, as the number of measles deaths worldwide reached a high of 134,200 cases in 2015 [29]. In China, the annual incidence has plummeted from 572.0 cases per 100,000 persons in 1960 to 7.6 cases per 100,000 persons in the 1990 with the aid of routine immunization. Yet, from 1995 the reported annual incidence started to rebound, peaking in 2005 with more than 120,000 reported cases [2,3]. In 2006, China endorsed the 2006–2012 National Action Plan, in which the Min- istry of Health set a goal for the reported annual incidence rate to be less than 1 per 100,000 persons in 2012, with the aim of measles elimination. The measles immunization strategy was based on a routine immunization program, integrated with supple- mentary immunization activities (SIAs) while reinforcing disease surveillance, outbreak control, hospital-acquired infection control, and health improvement schemes. http://dx.doi.org/10.1016/j.vaccine.2017.01.010 0264-410X/Ó 2017 Elsevier Ltd. All rights reserved. ⇑ Corresponding authors at: The Jockey Club School of Public Health Building, The Chinese University of Hong Kong, Prince of Wales Hospital, Shatin, New Territories, Hong Kong, China (M.H. Wang). E-mail addresses: haitian.wang@gmail.com (M.H. Wang), guanxh9999@163.com (X. Guan). 1 These authors contributed equally to this work. Vaccine 35 (2017) 1024–1029 Contents lists available at ScienceDirect Vaccine journal homepage: www.elsevier.com/locate/vaccine