In response to press enquiries on the latest recommendations on seasonal influenza vaccination (SIV) of the World Health Organisation (WHO), Secretary for Food and Health, Professor Sophia Chan, quoted findings from the Centre for Health Protection (CHP) that no significant antigenic changes had so far been detected in the local circulating influenza A(H3) virus strain. She added that it is too early to make any conclusion in terms of vaccine effectiveness.

No significant antigenic changes detected in local circulating H3 strain

The WHO regularly reviews the global epidemiology of influenza twice every year to recommend virus strains for inclusion in SIV for use in the northern hemisphere (NH) and southern hemisphere (SH), respectively, in accordance with the available data.

On 28 September, WHO released its recommended trivalent influenza vaccine (TIV) strains for the 2018 SH influenza season, including:

• An A/Michigan/45/2015 (H1N1) pdm09-like virus;
• An A/Singapore/INFIMH-16-0019/2016 (H3N2)-like virus; and
• A B/Phuket/3073/2013-like virus

The quadrivalent influenza vaccine (QIV) should contain the above trio and a B/Brisbane/60/2008-like virus.

"Regarding the H3N2 strain, we noted that the WHO recommended an A/Hong Kong/4801/2014 (H3N2)-like virus for NH while the above Singapore strain for SH. Both are antigenically similar with no antigenic drift," explained a spokesman for the CHP.

According to the WHO's report, influenza A(H3N2) and B viruses were predominant globally from February to September 2017. Studies also showed that recent H3N2 viruses had been better inhibited by a ferret antiserum raised against the above egg-propagated Singapore strain, as compared to other egg-propagated viruses (including the above Hong Kong strain). The Singapore strain was hence recommended as the vaccine strain.

"Our virus characterisation has so far detected no significant antigenic changes in the currently circulating H3 strain. Members of our Scientific Committee on Vaccine Preventable Diseases and its Working Group on Influenza Vaccination have been informed of the WHO report. We will continue to closely monitor the local and global influenza activities, predominating strains, vaccine effectiveness and relevant scientific literature," asserted the spokesman.

Insufficient data to conclude vaccine effectiveness

Regarding there are press reports questioning the effectiveness of the influenza vaccines purchased by the Hong Kong government, Sophia Chan emphasised CHP ’s preliminary analysis showed no significant variation in influenza virus strains. As such, it is too early to conclude on the effectiveness of the vaccine.

“I think at this point of time; the flu situation is still at a low level. If we are going to make any conclusion, it is too early, in terms of the vaccine effectiveness and also looking at the different strains. They have already communicated with their Scientific Committee (on Vaccine Preventable Diseases) and asked the Scientific Committee to read the World Health Organisation's report. They will continue to look at the strain to monitor whether there is any drifting of antigen. They will also look at the international literature and review the international flu situation,” shared Chan. She added vaccination is still the best way to prevent influenza. Hence, she urged citizens – children and senior citizens in particular – to get injected as soon as possible.

Professor Kwok-yung Yuen, Chair of Infectious Diseases at the University of Hong Kong (HKU)’s Department of Microbiology explained that although the vaccine for use this winter might be less effective against H3N2 influenza virus, it is still effective against H1N1 and influenza B virus. He encouraged patients with chronic diseases, the elderly and children to receive the vaccination.

Yuen further added that the mutation in a dominant flu strain had been observed earlier, and this might be one of the reasons for the surge in influenza cases this summer. He said there would usually be two outbreaks when there is H3N2 antigenic variation in the past, implying a possible H3N2 flu outbreak this winter. As it takes about six months to re-order the vaccine and the peak season for flu would have passed by then, Yuen said that there is no way for the government to remedy the situation now.

According to the spokesman for the CHP, "The latest surveillance data show that the local influenza activity remained at a low level and the positive percentage of seasonal influenza viruses was 2.79% in the week of 17 September. As the winter influenza season is yet to arrive, epidemiological and virological data, as well as scientific evidence, are still insufficient to forecast the possible predominating strains in the coming winter influenza season – and it is too early at this stage to predict whether the NH vaccine strains match with the future circulating viruses, the vaccine effectiveness and if SH vaccines should be considered.”

In addition, the CHP also highlighted that when vaccine strains closely match with circulating viruses, the vaccine efficacy typically ranges from 70 – 90%. Vaccination will also provide a certain degree of cross-protection even for different but related strains and will reduce disease severity particularly for high-risk groups including children, the elderly and chronic patients. Vaccination and strict personal and environmental hygiene remain the best means to prevent influenza. MIMS

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