Immunogenicity of Alternative Annual Influenza Vaccination Strategies in Older Adults in Hong Kong

Background: The typical vaccination strategy of annual administration with inactivated trivalent influenza vaccine (TIV) or quadrivalent influenza vaccine (QIV) may provide suboptimal protection to older adults in a location with prolonged periods of influenza activity because of the weaker immune response of older adults to vaccination and because of post-vaccination waning in protection over the course of a year. We hypothesize that in a subtropical or tropical location with prolonged circulation of influenza viruses, the higher antibody titers over years achieved after receipt of annual high-dose vaccine, MF59-adjuvanted vaccine or recombinant haemagglutinin (HA) vaccine, or different vaccination strategies of their combinations with or without the standard vaccine, might lead to greater protection than annual receipt of standard vaccines.

Aim: To test the immune profiles over time of older adults following different influenza vaccination strategies.

Design and subjects: Initially a 4-year immunogenicity study with a randomized controlled design among 2200 older adults aged 65-82 years. We will enroll participants who are willing to receive annual influenza vaccination from the general community including community centres and day-care centres. Eligible individuals will be randomly allocated to ten intervention groups (i.e. annual standard QIV, annual MF59-adjuvanted TIV, annual high-dose TIV, annual recombinant-HA QIV, and six combinations of their alternate annual use) consisting of four rounds of vaccination before each winter influenza season and followed up for 4 years. For each round of vaccination, blood samples for immunological tests will be collected before vaccination and 30 and 182 days after vaccination in all participants, and also at 7, 91 and 273 days after vaccination in a subset of 10% of the participants. Acute illnesses among participants will be monitored by active surveillance efforts during influenza seasons. The vaccine formulation in each round of vaccination will be updated for each season according to WHO recommendations.

Study extension: In years 5-8, all participants will receive the recombinant-HA QIV once per year before each winter influenza season, and their receipt of COVID-19 vaccines will also be recorded. Samples will be collected at the same timepoints to monitor immune responses to influenza vaccination and how responses are affected by prior vaccination history.

Main outcome measures: Antibody titers measured by haemagglutination-inhibition assays, which is an established correlate of protection, in addition to other measurements on humoral and cell-mediated immune responses in the ten intervention groups each year.

Inclusion criteria

• Adult aged 65-82 years attending ECC and EDC who has not received 2017/18 seasonal influenza vaccine and is willing to receive annual influenza vaccination

Exclusion criteria

• Individuals who show signs of dementia (do not pass the Mini-cog test under Appendix 1a: Recruitment Screening Log) or significant cognitive impairment and are not competent to give their consent.
• Individuals who report medical conditions not suitable to receive inactivated influenza vaccines, such as:

• Severe allergic reaction (e.g., anaphylaxis) after previous dose of any influenza vaccine; or to a vaccine component, including egg protein;
• Moderate or severe acute illness with or without fever after any previous influenza vaccination; or
• A history of Guillain-Barré syndrome (GBS) within 6 weeks of previous influenza vaccination.
• Individuals, who report medical conditions not suitable to receive intramuscular injection, such as:

• bleeding disorders
• habitually taking anticoagulants (with the exception of antiplatelets such as aspirin).
• Individuals who have any medical conditions not suitable to receive inactivated influenza vaccines as determined by a clinician.