Vaccine - (Page 2) Vaccine 26 (2008) 6157–6164 Contents lists available at ScienceDirect Vaccine journal homepage: www.elsevier.com/locate/vaccine A statistical model to estimate the impact of a hepatitis A vaccination programme ˜ Manuel Oviedo a,b,∗ , M. Pilar Munoz c,a , Angela Domínguez d,a , Eva Borras a , Gloria Carmona b a CIBER Epidemiología y Salud Pública (CIBERESP), Spain Directorate of Public Health, Generalitat of Catalonia, Spain c Department of Statistics and Operations Research, Technical University of Catalonia, Spain d Department of Public Health, University of Barcelona, Spain b article info abstract A program of routine hepatitis A + B vaccination in preadolescents was introduced in 1998 in Catalonia, a region situated in the northeast of Spain. The objective of this study was to quantify the reduction in the incidence of hepatitis A in order to differentiate the natural reduction of the incidence of hepatitis A from that produced due to the vaccination programme and to predict the evolution of the disease in forthcoming years. A generalized linear model (GLM) using negative binomial regression was used to estimate the incidence rates of hepatitis A in Catalonia by year, age group and vaccination. Introduction of the vaccine reduced cases by 5.5 by year (p-value < 0.001), but there was a significant interaction between the year of report and vaccination that smoothed this reduction (p-value < 0.001). The reduction was not equal in all age groups, being greater in the 12–18 years age group, which fell from a mean rate of 8.15 per 100,000 person/years in the pre-vaccination period (1992–1998) to 1.4 in the vaccination period (1999–2005). The model predicts the evolution accurately for the group of vaccinated subjects. Negative binomial regression is more appropriate than Poisson regression when observed variance exceeds the observed mean (overdispersed count data), can cause a variable apparently contribute more on the model of what really makes it. © 2008 Elsevier Ltd. All rights reserved. Article history: Received 11 March 2008 Received in revised form 18 July 2008 Accepted 30 August 2008 Available online 18 September 2008 Keywords: Hepatitis A vaccine Incidence Statistical models Log-linear models Overdispersion Count data Generalized linear models Negative binomial distribution Poisson distribution 1. Introduction Hepatitis A virus (HAV) infection is an acute self-limiting and often asymptomatic disease. However, in adults, the clinical presentation is characterised by acute onset of malaise, anorexia and abdominal pain followed by jaundice [1,2]. In 8–10% of cases, the disease may relapse after the acute phase. In some cases HAV may lead to extrahepatic complications (arthralgia, pancreatitis, vasculitis or glomerulonephritis) and fulminant hepatitis or death can occur [3]. The main transmission mechanism is the faecal-oral route, either through person–person contact or by ingestion of contaminated foods or water [4]. Hepatitis A can also be acquired, although more rarely, by transfusion of blood or blood-products, sexual contacts, dental procedures and tattooing [5,6]. ∗ Corresponding author at: CIBER Epidemiología y Salud Pública (CIBERESP), Parc de Recerca Biomèdica de Barcelona, c/ Dr. Aiguadé 88, 08003 Barcelona, Spain. Tel.: +34 935513666; fax: +34 935517506. E-mail address: manel.oviedo@gencat.cat (M. Oviedo). 0264-410X/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.vaccine.2008.08.066 In developed countries, the main high-risk groups are people travelling to endemic countries, male homosexuals, injecting and noninjecting drug users, persons with chronic liver disease, persons with clotting factor disorders and persons who work with HAV in search settings [7]. However, infection is not limited to these groups and the source of infection is not determined in about half the cases [1,6]. Hepatitis A usually presents epidemic waves that are repeated in cycles that vary according to the circulation of the virus [1,5]. In countries of low endemicity, such as Spain, hepatitis A virus infection has shifted to older age groups, implying a more severe clinical course [8]. The availability of an inactivated vaccine of proven immunogenicity and protective efficacy [9,10] led to the introduction of the vaccination of risk groups in Catalonia in 1995. The results showed that the impact of vaccination of these risk groups on the global incidence was small, as shown by other studies [11]. From the end of 1998, hepatitis A vaccine was administered routinely at 12 years of age in the form of three injections of hepatitis A + B vaccine, at 0, 1 and 6 months. The effectiveness, i.e. the extent to which a specific intervention, when deployed in the field in routine circumstances, does what it is intended to do for a specific population [12] of the http://www.sciencedirect.com/science/journal/0264410X http://www.elsevier.com/locate/vaccine http://dx.doi.org/10.1016/j.vaccine.2008.08.066
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