Atlas Monitor | 23 Oct 2014
The efficacy of the seasonal flu vaccine is contingent upon vaccine developers matching the developed vaccine with the circulating strain; a process that is analogous to forecasting the weather according to NZ National Influenza Centre director Sue Huang.
Eight per cent of flu sufferers in NZ this winter had a strain not covered by this season’s flu shot. It has been characterized as a “rouge” strain that has raised questions over the overall efficacy of flu vaccination.
“We don’t know the exact figure [of] how many people [were] immunized” that got the flu this season claims Huang.
This season’s flu vaccine is alleged to protect against the H1N1 A strain; however, left people vulnerable to the H3N2 which mutates so frequently vaccine developers cannot keep pace given the six month lead in time for vaccine development.
Although the nature of the H3N2 is known to vaccine developers the extent of the “drift” in mutation took many by surprise. Often times the flu strain vaccines are developed for do not match the circulating strain due to such mutations thereby reducing protection, suggests Huang.
According to Huang complex factors determine the overall efficacy of flu vaccination. Strain matching as well as the overall health status of an individual and in particular any underlying medical condition are such factors.
The heterogeneous composition of the population also make it very difficult to develop a vaccine with maximum efficacy. Vaccine effectiveness sits between 50 and 80 per cent according to Huang. This season’s effectiveness was 60 per cent.
However some detail and nuance is necessary to understand exactly what “effectiveness” means. Effectiveness is defined in terms of whether or not someone is hospitalized or is compelled to visit their GP; not whether or not someone exhibits flu symptoms.
This season’s effectiveness statistics suggest the flu vaccine was 54 per cent effective in preventing hospitalizations and 67 per cent effective in preventing people from having to visit the doctor.
Statistics for exhibiting any symptoms whatsoever do not appear to be available if even at all calculable.
The H3N2 was the dominant strain in the influenza outbreak in New South Wales this year rendering the vaccine generally ineffective.
The World Health Organization advises each year on which flu strain vaccines ought to be developed to vaccinate against. Next year’s flu vaccine would include the same H1N1 strain as well as a strain similar to the H3N2.
The number people hospitalized or visiting their doctor with flu symptoms was the third lowest level this year since 1992.
