Dr Tam Yat-hung obtained his medical degree at the Chinese University of Hong Kong, and his master degrees in public health at HKU and in infectious diseases at the London School of Hygiene and Tropical Medicine. He is fellows of the Hong Kong Academy of Medicine in the specialty of community medicine and of the Faculty of Public Health of the Royal Colleges of Physicians of UK. Before joining the School of Public Health, he worked in different services of the Centre for Health Protection and the Primary Care Office of the Department of Health, the Government of the HKSAR. His practice mainly focused on communicable disease surveillance, epidemiological investigation of disease outbreak and implementation of control and preventive measures, with special interest in field epidemiology. He is currently involved in the public health education for undergraduate and postgraduate students and training of public health specialists. His main research interests include vaccine immunogenicity, efficacy and effectiveness; vaccination strategies; infectious disease surveillance and epidemiology.
Factors affecting responses to twice-annual seasonal influenza vaccination in older adults in Hong Kong
Background: Hong Kong as many other tropical and subtropical areas experiences more prolonged influenza seasons every year. Annual influenza vaccination of older adults is recommended to reduce disease burden. In 2014–2015, Hong Kong experienced a substantial A(H3N2) winter epidemic with a mismatched vaccine. Local authorities procured and administered to older adults the 2015 southern hemisphere influenza vaccine, which included an updated and matching H3N2 strain. We hypothesized that twice-annual influenza vaccination might provide older adults with improved immunity against influenza, and investigated the potential factors affecting the responses to NH IIV in 2015/16.
Methods: Older adults aged ≥75 years who were receiving SH IIV under the government’s special vaccination campaign were enrolled in summer 2015 and followed through to winter 2015-16 when they received NH IIV (twice-annual vaccination). Pre- and post-vaccination sera were collected for measuring antibody titres against vaccine strains. A separate group of older adults who received NH IIV in winter 2015-16 without prior receipt of the SH IIV in summer 2015 were enrolled as comparison group (once-annual vaccination). Potential factors associated with immune and personal responses to NH IIV in both groups were analyzed.
Results: We enrolled 978 older adults with 470 vaccinations for summer 2015 and 827 vaccinations for winter 2015–2016. Recipients of SH IIV had higher antibody titers by the hemagglutination inhibition assay against all 3 vaccine strains in SH IIV. When receiving the NH IIV in winter 2015-16, the SH IIV recipients had higher prevaccination titres but lower postvaccination titres, compared to those who had not received the SH IIV. At the programme level, recipients to both SH IIV and NH IIV were 3 years older on average and more likely to have received influenza vaccination in the prior 2 years.
Conclusions: We observed some reductions in immune responses in the twice-annual vaccination group compared with the once-annual vaccination group, in the context of unchanging vaccine strains, while protection was likely to have been improved during the summer and autumn for the twice-annual vaccination group due to the continued circulation of the A/Switzerland/9715293/2013(H3N2) virus. Regular vaccination may be a behavioral factor for acceptance to additional vaccination for emerging illnesses.