Drugs During Pregnancy [back] Vaccination and pregnancy
Misrepresenting smallpox vaccine in pregnancy findings
Sunday, November 1, 2009
Re:
Evaluation of Preterm Births and Birth Defects in Liveborn Infants of US
Military Women Who Received Smallpox Vaccine. Margaret A. K. Ryan, Gia R.
Gumbs, Ava Marie S. Conlin, Carter J. Sevick, Isabel G. Jacobson, Katherine J.
Snell, Christina N. Spooner, Tyler C. Smith, for the Department of Defense Birth
and Infant Health Registry Team. Birth Defects Research (Part A) 82:533-539
(2008)
Last April I wrote a
detailed critique in this blog of a paper by Ryan et al. on
anthrax vaccinations in pregnancy and
birth defects. In the
paper I am discussing today, Ryan has studied birth defects in military
servicewomen who received smallpox
vaccine during pregnancy. In fact, there is likely significant overlap between
the two groups of women Ryan studied, since both anthrax and smallpox vaccines
have been given to all service members deploying to
Iraq, Afghanistan and Korea since 2003.
The findings of the two studies are, no surprise, very similar: women receiving
smallpox vaccine during the first trimester of pregnancy had a rate of major
birth defects in their offspring of 4.5%, while the offspring of women
vaccinated post-pregnancy had a major birth defect rate of only 3.2%.
The birth defect rate is therefore 40% higher if
the mother was vaccinated during the first trimester.
The paper fails to explain significant problems with the military database,
whose accuracy was previously studied by Ryan and discussed in my
earlier blog. The sensitivity for identifying anthrax vaccinations in this
database was less than 70%. How well does it correctly identify smallpox
vaccinations?
Only 30 of 672 women vaccinated in the first trimester had infants born with
major birth defects. So even though the birth defect rate was 40% higher, it did
not achieve statistical significance. The authors then
incorrectly concluded that, "smallpox
vaccine, when inadvertently administered to pregnant women, is
not associated with preterm delivery or
birth defects in liveborn infants."
What would a true scientist have done with these results?
First, a true scientist would not have misrepresented the data. To be accurate,
the paper should have said that a sizeable increase in birth defects was found,
but given the numbers involved, did not reach statistical significance.
Second, the paper should have pointed out that a similar effect had been found
for anthrax vaccination, in a larger but overlapping cohort of servicewomen, by
three of the same authors. The two
vaccines administered together probably caused confounding, making it impossible
to determine how much each vaccine may have contributed to the birth defects
(which were statistically significant at the 0.05% level for some of the anthrax
findings).
Third, a true scientist would have expanded the sample size, to try and achieve
statistical significance. This would have simply entailed using the same
database over a longer duration. The paper was submitted in 2008, so there was
time to obtain additional data on servicewomen vaccinated since 2004. Of course,
then the result might no longer be used to support military vaccine policy.