The highest rates of influenza-related morbidity and mortality occur in people aged over 65 years infected with A(H3N2) strains. There is a direct relationship between seasons when an influenza A(H3N2) strain is the predominant strain in circulation and increased hospitalisations with influenza-associated respiratory and circulatory conditions.11
No link has been shown between levels of circulation of influenza B viruses and excess mortality or seasonal surges in hospitalisations.12
When I first diagnose a patient with diabetes, I don’t ask them if they feel like taking insulin. Similarly, in winter when flu vaccine becomes available, I tell patients it has arrived and that I will give it to them while they are there. I get very few discussions or refusals. This is best practice.
Influenza vaccine responses in older people
Immunosenescence, an age-related decline in immune function, impairs the ability of older adults to fight natural infections and also results in suboptimal immune responses to influenza vaccines.13 Both adaptive and innate immunity decline with increasing age in the population aged over 65 years.
Although some studies have found little protection from the use of standard influenza vaccine in this older age group, conclusions are clouded by the mismatch in some years between viral strains in the vaccine and those circulating in the population, and the different outcomes evaluated. Indeed, studies have shown that inactivated influenza vaccine may halve the incidence of laboratory-proven and clinical influenza.14 Even when vaccination failed to stop infection, it did decrease the severity of disease, as evidenced by lower hospitalisation rates and fewer admissions to intensive care units.15
Influenza vaccine effectiveness in older people varies with the circulating strain, being lower in years when influenza A(H3N2) predominates. Older adults have the poorest antibody-mediated immune responses to the A(H3N2) components of vaccines and also display lower cellular immunity to influenza A(H3N2).
Influenza vaccination is recommended and funded in Australia for all people aged 65 years and over. Previously, the most widely used influenza vaccines were trivalent formulations of inactivated haemagglutinin and neuraminidase antigens representative of the predominant A(H1N1), A(H3N2) and B strains, using the selected strains recommended by WHO for each season. More recently, both influenza B strain lineages (B/Yamagata and B/Victoria) have been included in new quadrivalent influenza vaccines. The WHO recommended strains for the trivalent and quadrivalent influenza vaccines for the 2019 southern hemisphere influenza season are listed in Box 1.
Quadrivalent influenza vaccines may have benefit in children, who experience the highest burden of influenza B. However, they are relatively less advantageous for older people, in whom most serious disease is attributable to influenza A(H3N2), with little disease or serious disease being due to influenza B. Adding additional lineage coverage for influenza B to vaccines for older people would be of little benefit, as it has no impact on overcoming immunosenescence and improving effectiveness against influenza A disease.
Enhanced vaccines are required to provide adequate protection in older people.16,17 The Australian Government funded two new enhanced vaccines for people aged 65 years and over for the first time in 2018: an adjuvanted trivalent vaccine and a high-dose trivalent vaccine. Both vaccines showed improved effectiveness in real-world studies and elicited greater antibody responses in clinical trials. In September 2017, the Chief Medical Officer of Australia issued guidance on the importance of vaccinating older patients and also of using the new enhanced vaccines in this specific age group.18 Vaccines available in Australia in 2019 are shown in the Table.
Enhanced influenza vaccines
Adjuvanted influenza vaccine
The adjuvanted trivalent influenza vaccine not only enhances the magnitude of the immune response but also broadens the response to improve protection during years when vaccine strains do not match circulating viruses. The adjuvant MF59 is an oil-in-water emulsion based on squalene, which enhances both antigen presentation and T-cell priming. Importantly, a number of international studies of adjuvanted influenza vaccine found a significant increase in the response to influenza A(H3N2) in people aged over 65 years.19,20 In 2019, the adjuvanted trivalent influenza vaccine is recommended as one of the two preferred vaccines for use in older people and is funded under the National Immunisation Program (NIP).