Immunogenicity of 2011 Trivalent Influenza Vaccine among Healthcare Workers in Nakhon Ratchasima

Abstract

In Thailand, influenza vaccination program was launched in 2004. During 2004-2007, the influenza vaccine was recommended for only healthcare workers (HCWs) and gradually expanded to include other subpopulation groups in 2008. Although the healthcare workers acquire an annual vaccination, outbreaks of influenza among healthcare workers had been reported frequently. This study was conducted to determine influenza seroconversion rate and protective level of immunity of the 2011 trivalent influenza vaccine. This was a cross-sectional conducted to determine immunogenicity. We collected serum samples from vaccines for Haemagglutination inhibition (HI) test both before and 30 days after vaccination. Geometric mean titer, influenza seroconversion rate and protective level of immunity were estimated. It was found that among 98 healthcare workers enrolled in immunogenicity study, highest antibody geometric means titer was in A/Perth/16/2009/H3N2 (52.3), followed by B/Brisbane/60/2008 (38.2) and A/California/7/2009/H1N1 (35.4). The high percentages of influenza seroconversion were observed in A/Perth/16/2009/H3N2 (37.7%), A/California/7/2009/H1N1 (35.7%) and B/Brisbane/60/2008 (15.3%). The percentage of vaccines who reached protective level of immunity after receiving vaccine was highest in A/Perth/16/2009/H3N2 (70.4%), followed by B/Brisbane/60/2008 (66.3%) and A/California/7/2009/H1N1 (57.1%). In conclusions, antibody geometric mean titer and influenza seroconversion after receiving 2011 trivalent influenza vaccine was highest in A/Perth/16/2009/H3N2. The vaccine raised herd immunity level in all subtypes of influenza virus to over 50%.