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When Studies Collide

 

 

# 3810

 

 

If you are a frequent reader of scientific studies, you are no doubt aware that science isn’t always neat, tidy, orderly and clear.   We get conflicting data, and analysis, all of the time.  

 

Sorting out what is true . . . or at least likely to be true . . . is a continual challenge for the reader.

 

Up until recently the poster child for scientific ambiguity has been the debate over whether surgical masks are effective in protecting Health Care Workers from influenza. We get studies that say they work, and studies that say they are practically useless.  

 

But now, at least temporarily, the debate over whether seasonal flu vaccines offer protection against the novel H1N1 virus, or increase your odds of catching it, takes center stage.

 

In the past couple of weeks we’ve heard a good deal about `The Canadian Problem’, an as yet unpublished study that links getting last year’s seasonal flu vaccine with an increased likelihood of catching the novel H1N1 virus.

 

This study is still undergoing peer review, but its impact has already been felt across much of Canada.   Many provinces have curtailed their seasonal flu vaccination programs for those under the age of 65, out of concern that taking the seasonal vaccine may exacerbate the pandemic.

 

The WHO and the CDC meanwhile say they’ve seen no evidence to support this supposed link. They continue to recommend taking the seasonal flu vaccine.

 

As if this weren’t confusing enough, now we get a study published in the BMJ (British Medical Journal) that suggests exactly the opposite - that getting the seasonal flu vaccination may be slightly protective against the swine flu.

 

This also runs contrary to what we’ve heard from the CDC, which  maintains that the seasonal vaccine is not expected to offer any protection against the novel H1N1 swine flu virus.

 

 

 

Partial protection of seasonal trivalent inactivated vaccine against novel pandemic influenza A/H1N1 2009: case-control study in Mexico City

Published 6 October 2009, doi:10.1136/bmj.b3928

Objective To evaluate the association of 2008-9 seasonal trivalent inactivated vaccine with cases of influenza A/H1N1 during the epidemic in Mexico.

Design Frequency matched case-control study.

Setting Specialty hospital in Mexico City, March to May 2009.

Participants 60 patients with laboratory confirmed influenza A/H1N1 and 180 controls with other diseases (not influenza-like illness or pneumonia) living in Mexico City or the State of Mexico and matched for age and socioeconomic status.

Main outcome measures Odds ratio and effectiveness of trivalent inactivated vaccine against influenza A/H1N1.

Results Cases were more likely than controls to be admitted to hospital, undergo invasive mechanical ventilation, and die. Controls were more likely than cases to have chronic conditions that conferred a higher risk of influenza related complications. In the multivariate model, influenza A/H1N1 was independently associated with trivalent inactivated vaccine (odds ratio 0.27, 95% confidence interval 0.11 to 0.66) and underlying conditions (0.15, 0.08 to 0.30). Vaccine effectiveness was 73% (95% confidence interval 34% to 89%). None of the eight vaccinated cases died.

 

 

Conclusions Preliminary evidence suggests some protection from the 2008-9 trivalent inactivated vaccine against pandemic influenza A/H1N1 2009, particularly severe forms of the disease, diagnosed in a specialty hospital during the influenza epidemic in Mexico City.

 

 

So, the reader is now in the unenviable position of having to decide decide between 3 different stances. 

 

  1. Taking the Seasonal Flu vaccine may increase your odds of getting the pandemic flu (Canadian Unpublished Study)
  2. Taking the Seasonal Flu vaccine may decrease your odds of getting the pandemic flu (BMJ study)
  3. Taking the seasonal flu vaccine has no effect on your risks of acquiring the pandemic flu (CDC)

These are not trivial matters.   Lives, and global public health policies, hinge on sorting this all out as quickly (and scientifically accurately) as possible. This kind of ambiguity and debate, while fascinating to us disease-geeks-types, no doubt breeds apathy and mistrust among the public.

 

This BMJ study is, admittedly, on a very small number of patients.  The Canadian study is unpublished, and so we have very little to go on.   So, as always, Caveat Lector.

 

As far as which study is right, I’ve no idea.

 

Since I’ve already gotten my seasonal shot, I’m hoping it’s either option #2 or #3. 

 

But I suppose only time will tell.

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Study
Pandemic
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