In a strong piece at CNN online yesterday, Jen Christensen points out that no European countries expect the entire population to be immunized against flu — unlike the US, where everyone over the age of 6 months is urged to get flu vaccine every year.
A few possibilities:
1. Public health benefit?
No. Over the past twenty years, flu-vaccine coverage — the proportion of the population that is immunized — has been going up progressively. But flu hospitalization and mortality rates have been basically constant. If mass immunization had any public health value, those rates should go down as coverage goes up
(A technical note: this means that coverage remains below the threshold needed to reduce influenza transmission population-wide, i.e., it isn’t high enough for herd immunity. But that’s the point. In order to be of public health benefit, flu vaccine would have to be accepted by almost everybody, every year. And even that might not be enough: For a nice explanation of why the efficacy of flu vaccine is limited, see Vincent Racaniello’s blog post.)
2. Exceptional efficacy of the vaccine?
No. Based on an observational study of acute respiratory illness patients published this month, the effectiveness of this year’s flu vaccine is 55% against illness caused by influenza type A (which accounts for about 80% of flu cases). Effectiveness is 70% against type B. Overall, the chances of being protected against symptomatic flu are less than two out of three.
Jefferson and colleagues found that the overall efficacy of flu vaccines at reducing influenza A or B infection in children aged 2-16 is only about 65%, and that inactivated vaccines (i.e., the usual injection) had little impact on serious illness or hospitalization from flu-like conditions in this age group.
As with this month’s observational study, Jefferson et al.’s meta-analysis of multiple studies on flu immunization found that the inactivated vaccine had about 73% efficacy at preventing infection in healthy adults — but that efficacy can be as low as about 50% in years when the vaccine isn’t well-matched to the season’s circulating viruses.
Importantly, the Jefferson studies found that effectiveness of immunization — the prevention of serious illness or hospitalization from influenza-like illness — is very low.
There’s no sound public health rationale for encouraging everyone to be immunized against flu every year.
People who are likely to develop serious complications if they are infected can benefit from immunization. But for most of us, immunization only reduces (by two-thirds) the already rather small chance of infection with influenza. And it doesn’t protect us much from serious respiratory illness during flu season.
I commented in 2011 on public officials striving to help pharmaceutical companies profit from flu fears. And that’s what we’re seeing again this season — with exaggerated warnings and declarations of flu emergencies. Even though the latest national summary from CDC shows that less than 30% of all influenza-like illness is actually caused by flu this season — and that’s likely an overestimate, since it’s based on testing of more severe cases of acute respiratory illness. And the surveillance data suggest that the season’s flu outbreak might already be past its peak.
Get immunized against flu if you’re worried. But keep in mind that vaccination against flu is not going to help the public’s health, and it isn’t highly likely to help yours — it’s primarily your contribution to the profits of Sanofi-Pasteur, Novartis, GSK, or Merck.