Kriengkrai Prasert1, Jayanton Patumanond2, Prabda Praphasiri3, Supakit Siriluk4, Darunee Ditsungnoen5, Malinee Chittaganpich6, Fatimah S Dawood7, Joshua A Mott8, Kim A Lindblade8. 1. Nakhon Phanom Provincial Hospital, Nakhon Phanom, Thailand; Thammasat University, Pathum Thani, Thailand. 2. Thammasat University, Pathum Thani, Thailand. 3. Influenza Program, Thailand MOPH-US CDC Collaboration, Nonthaburi, Thailand. Electronic address: hpu3@cdc.gov. 4. Health Technical Office, Thai Ministry of Public Health, Nonthaburi, Thailand. 5. Influenza Program, Thailand MOPH-US CDC Collaboration, Nonthaburi, Thailand. 6. National Institute of Health, Thai Ministry of Public Health, Nonthaburi, Thailand. 7. Influenza Division, US Centers for Disease Control and Prevention, Atlanta, GA, USA. 8. Influenza Program, Thailand MOPH-US CDC Collaboration, Nonthaburi, Thailand; Influenza Division, US Centers for Disease Control and Prevention, Atlanta, GA, USA.
Abstract
BACKGROUND: We conducted a two-year prospective cohort study to measure the effectiveness of trivalent inactivated influenza vaccine (IIV3) to prevent laboratory-confirmed influenza among community-dwelling Thai adults aged ≥65 years during 2015-16 and 2016-17 influenza seasons. METHODS: In 2015, we enrolled a cohort of 3220 participants. Trained health volunteers collected baseline data and followed participants for two years with weekly surveillance for new or worsened cough with self-collection of nasal swabs. Vaccine effectiveness (VE) was estimated as 100% × (1- rate ratio of rRT-PCR -confirmed influenza) among vaccinated versus unvaccinated participants. Propensity score stratification was used to reduce differences between vaccinated and unvaccinated participants associated with access to and receipt of IIV3. FINDINGS: During 2015-16 and 2016-17, 1666 (52%) and 1498 (48%) participants received IIV3, respectively. The overall incidence of influenza during the two seasons was 14.3/1000 person-years among vaccinated participants and 20.2/1000 person-years among unvaccinated participants. VE was -4% (95% confidence interval [CI], -83%-40%) during 2015-16 when there was poor antigenic match between the dominant circulating A/H3N2 viruses and the vaccine strain, and 50% (95% CI, 12-71%) during 2016-17 when circulating and vaccine strains were well-matched. Of all three influenza subtypes in both years, significant protection was observed only against Influenza A/H3N2 during 2016-17 (VE, 49%; 95% CI, 3-73%). INTERPRETATION: During a season with well-matched circulating and vaccine strains, IIV3 was moderately effective against laboratory-confirmed influenza among older adults in Thailand. Published by Elsevier Ltd.
BACKGROUND: We conducted a two-year prospective cohort study to measure the effectiveness of trivalent inactivated influenza vaccine (IIV3) to prevent laboratory-confirmed influenza among community-dwelling Thai adults aged ≥65 years during 2015-16 and 2016-17 influenza seasons. METHODS: In 2015, we enrolled a cohort of 3220 participants. Trained health volunteers collected baseline data and followed participants for two years with weekly surveillance for new or worsened cough with self-collection of nasal swabs. Vaccine effectiveness (VE) was estimated as 100% × (1- rate ratio of rRT-PCR -confirmed influenza) among vaccinated versus unvaccinated participants. Propensity score stratification was used to reduce differences between vaccinated and unvaccinated participants associated with access to and receipt of IIV3. FINDINGS: During 2015-16 and 2016-17, 1666 (52%) and 1498 (48%) participants received IIV3, respectively. The overall incidence of influenza during the two seasons was 14.3/1000 person-years among vaccinated participants and 20.2/1000 person-years among unvaccinated participants. VE was -4% (95% confidence interval [CI], -83%-40%) during 2015-16 when there was poor antigenic match between the dominant circulating A/H3N2 viruses and the vaccine strain, and 50% (95% CI, 12-71%) during 2016-17 when circulating and vaccine strains were well-matched. Of all three influenza subtypes in both years, significant protection was observed only against Influenza A/H3N2 during 2016-17 (VE, 49%; 95% CI, 3-73%). INTERPRETATION: During a season with well-matched circulating and vaccine strains, IIV3 was moderately effective against laboratory-confirmed influenza among older adults in Thailand. Published by Elsevier Ltd.