Influenza Vaccine: Calling the shots
To stay effective, flu vaccines must keep pace with rapidly mutating viruses.
This year marks 100 years since the Spanish influenza pandemic that infected one quarter of the world’s population and killed around 50 million people. Even in non-pandemic years, influenza is responsible for more than half a million deaths worldwide1 and causes a significant burden on health systems. This is despite the development of an influenza vaccine in the late 1940s and the establishment of a global surveillance network in the early 1950s to identify circulating virus strains for inclusion in upcoming vaccines.2
The success of global surveillance varies. Vaccine effectiveness is impacted by antigenic “drift” in the virus's circulating strains. Antigens are molecules on the viral surface that trigger immune defences. The body remembers these antigens allowing a rapid response when the pathogen is next encountered.
Influenza antigens undergo rapid change—within a few years of encountering an influenza strain, the body does not recognise it as the same pathogen, and we become vulnerable to the flu again.3 The seasonal vaccine must account for this antigenic drift to be effective.
In the 2017/18 influenza seasons, effectiveness against the influenza A (H3N2) strain of the virus ranged between 10 and 25 per cent, while effectiveness against influenza A (H1N1) and Influenza B was greater. Unfortunately, Influenza A (H3N2) was the predominant circulating virus. However, even with low effectiveness, vaccination is estimated to have prevented thousands of hospitalisations.4 In 2018, the Bill and Melinda Gates Foundation announced a fund to develop a universal influenza vaccine to overcome this seasonal uncertainty.5
The 2018 seasonal influenza vaccine in New Zealand and Australia is quadrivalent (acting against four viral strains) and uses inactivated subunits of the viruses, which cannot cause disease.6 The four influenza viruses include two B-strains and two type A. The part of the vaccine targeting the H3N2 strain has been altered to more closely reflect the circulating strain from the Northern hemisphere’s winter season, so should be more effective than last year’s vaccine.
Recent evidence suggests the effectiveness of vaccines wanes during the influenza season, prompting calls to delay vaccination until closer to the start of the season. However, variations in when the influenza season starts (mid-May to early June) and peaks (mid-June through to mid-August,7 make it impossible to determine an ideal window for vaccination. The risk of people missing vaccination or being too late outweighs that of waning protection, and vaccination should be encouraged from the time the vaccine becomes available.8
Health care workers are twice as likely to be infected with influenza than the general population, can carry the virus without becoming sick, and are in greater contact with vulnerable populations—pregnant women, children under 5, the elderly, and those with increased vulnerability e.g. compromised immunity, asthma, and chronic heart or lung disease.9
Nurses have a professional responsibility to protect our patients from influenza but the average coverage within district health board staff in New Zealand was only 66 per cent in 2017 (range 51 to 84 per cent). Coverage of 80 per cent is necessary to provide herd immunity.10
You might like to complete the learning activity: Virsues and Pandemics on the CPD4nurses site.