April 25, 2010 18:00
Sara Powell asks why researchers used the Hepatitis A vaccine for the control group instead of just placebo in the JAMA study I recently blogged here about. Great question, Sara.
The methods section states that hepatitis A vaccine was chosen for the control group "because it is well tolerated and provides a potential health benefit given reported outbreaks of Hepatitis A on Hutterite colonies" (the communities studied). Since studies have to be approved by the Institutional Review Board, which looks hard to protect human subjects, an intervention with potential benefit may have seemed like a better choice than a shot of nothing as the placebo.
I contacted the lead author, Dr Loeb, who told me the potential benefit was the main reason. He also pointed out that Hepatitis A vaccine was previously used as a control in a CDC pediatric influenza study, so there was good precedent.
Here's my take: you can use placebo (like salt water) or you can choose something that might more closely mimic the experience of an actual vaccine. In general, scientists try to keep as much as possible the same for the two groups, except for the precise thing they are testing - in this case, influenza immunity itself.
For example, let's imagine that getting the Hepatitis A vaccine revved up the immune system, even in a non-specific way, such that vaccine recipients were able to fight off other infections more effectively. If this were true, it would potentially affect the study, because even those receiving Hepatitis A might be less likely to get (or get sick from) influenza, and therefore less likely to pass it on. It didn't turn out that way - the herd immunity was specific to those in the community where kids got influenza vaccine - but using actual vaccine for the control group in the study design might make it even harder to show that difference. It's my own curiosity talking here, so I asked the lead author, Dr Loeb, to join our conversation. Let's see if he chimes in!