# 5222
Influenza vaccines are generally expected to be around 70% to 90% effective on healthy adults under the age of 65 in years when the vaccine is a good match for the influenza strains that are in circulation.
But since vaccine strains must be selected roughly six months in advance, and the viral landscape can change over that time, not every year’s vaccine is fully on target.
Additionally, some vaccine strains may not produce as robust of an immune response as others.
So we really don’t know how effective a vaccine is going to be until after the flu season is over.
Today, we’ve a study that appears in PLoS Medicine that looks at that very topic, with regards to the 2009 H1N1 pandemic vaccine used in Europe.
The short answer to the question is that researchers found evidence that suggests the 2009 Pandemic vaccine worked pretty darn well.
Here’s the `money quote’:
. . . one dose of the pandemic vaccines used in the participating countries conferred good protection (65.5%–100% according to various stratifications performed) against pH1N1 in people who attended their practitioner with influenza-like illness, especially in people aged <65 years and in those without any chronic disease.
But of course, there’s more to the study than that.
You can read the whole thing (it’s open access) at the link below. Some excerpts from the press release follow.
Valenciano M, Kissling E, Cohen J-M, Oroszi B, Barret A-S, et al. 2011 Estimates of Pandemic Influenza Vaccine Effectiveness in Europe, 2009–2010: Results of Influenza Monitoring Vaccine Effectiveness in Europe (I-MOVE) Multicentre Case-Control Study. PLoS Med 8(1): e1000388. doi:10.1371/journal.pmed.1000388
Press release from PLoS Medicine
Pandemic H1N1 influenza vaccine effective in 2009-10 flu season
One dose of the pandemic flu vaccines used in seven European countries conferred good protection against pandemic H1N1 influenza in the 2009-10 season, especially in people aged less than 65 years and in those without any chronic diseases. These findings from a study funded by the European Centre for Disease Prevention and Control (ECDC) and coordinated by EpiConcept, Paris, France, published in this week's PLoS Medicine, give an indication of the vaccine effectiveness for the influenza A (H1N1) 2009 strain included in the 2010-11 seasonal vaccines.
The authors conducted a multi-centre case-control study based on practitioner surveillance networks from seven countries—France, Hungary, Ireland, Italy, Romania, Portugal and Spain. Patients consulting a participating practitioner for influenza-like-illness had a nasal or throat swab taken within eight days of symptom onset. Individuals were considered vaccinated if they had received a dose of the vaccine more than 14 days before the date of onset of influenza like illness and unvaccinated if they were not vaccinated at all or if the vaccine was given less than 15 days before the onset of symptoms.
The authors analysed pandemic influenza vaccination effectiveness in those vaccinated less than 8 days, those vaccinated between, and including, 8 and 14 days and those vaccinated more than 14 days before onset of symptoms compared to those who had never been vaccinated. The authors then used statistical models to measure the effectiveness of pandemic influenza vaccine according to three age groups (< 15, 15-64, and 65+ years of age) and the presence of chronic diseases. These results obtained during the late phase of the pandemic suggest good protection with the pandemic H1N1 vaccine (vaccine effectiveness estimates between 65% and 100%). The findings also suggest that the 2009-10 seasonal influenza vaccine (as opposed to the pandemic H1N1 vaccine) did not protect against pandemic H1N1 influenza illness.
The authors said: "The late availability of the pandemic vaccine and subsequent limited coverage with this vaccine hampered our ability to study vaccine benefits during the outbreak period." They added: "Future studies should include estimation of effectiveness of the new trivalent vaccine in the 2010-2011 season, when vaccination will occur before the influenza season starts."
Studies such as these always have limitations, hence words like `estimates’ and `suggests’ are utilized in their findings.
In a perfect world, researchers would conduct large blinded RCTs (Randomized Control Trials) to get the best possible evidence, but that often isn’t feasible (or ethical).
And even RCTs can be flawed (see RCTs: All That’s Gold Standard Doesn’t Glitter).
So researchers must find less direct ways to measure the effectiveness of a vaccine.
In this case, they looked at H1N1 infection rates among patients who reported an ILI (influenza-like-illness), used interviews and questionnaires to determine their immunization status, and then applied sophisticated (and well above my pay grade) statistical analysis to come up with their results.
While the authors believe their methodology was sound, and that the evidence suggests the pandemic vaccine worked remarkably well, they do offer the following caveats:
We believe these results should be interpreted with caution for reasons including the late timing of the studies relative to pandemic vaccine rollout, low incidence of medically attended H1N1 illness, low vaccine coverage and potential biases due to the test-negative design, confounding factors, and missing values.
Caveats aside, this study indicates the vaccine was pretty effective. Good news, particularly when you add in the safety profile of the pandemic vaccine, which has proved comparable to the seasonal flu shot.
Given the trepidation by the public over the pandemic vaccine, its actual track record over time has proved very reassuring.
While I was working on this blog entry, Lisa Schnirring and Robert Roos published an excellent overview of this study on the CIDRAP site, including some expert opinion regarding the study’s methodology.
European study says H1N1 vaccine offered good protection
Lisa Schnirring and Robert Roos
Staff Writers
