A Philadelphia Inquirer story covers the latest Centers for Disease Control and Prevention study that asked how much pregnant mothers’ vaccines for pertussis — more commonly called whooping cough — helped their babies in the first 2 months of life. As the story notes, this is a compelling research question because a mother’s antibodies transfer to her newborn, offering a temporary form of protection after birth (until a baby develops her own immunities after receiving the appropriate childhood vaccines).
The piece does a good job of explaining the core research question and the stakes. However, while the key numbers used in the story appear straightforward, they are derived from a statistical method that is not, i.e. odds ratios. It would have helped to see the study’s results characterized in more absolute terms, and the quality of its evidence discussed in slightly more detail.
Pertussis, or whooping cough, can be a deadly disease for non-immunized infants. Because infections are increasing in the U.S., it is important to look at various public health interventions to prevent new cases from occurring. This information is newsworthy and especially relevant for pregnant mothers, as they may have the ability to lower their child’s risk of getting infected during a vulnerable time in their baby’s life.
The cost of a Tdap vaccine for mothers isn’t noted, and it should be: It’s free under any insurance plan, per the Affordable Care Act (sometimes called “Obamacare”). For anyone paying out of pocket, though, it’s about $64 per dose.
The story notes that a mother getting a Tdap vaccine in her third trimester cuts the risk by 78% of a baby less than 2 months old of getting pertussis — and that even when a mother-vaccinated baby did get the disease, 90% of cases were mild.
However, the story cites risk numbers calculated from odds ratios — a statistical concept that most readers are not familiar with. A clearer way to present the results would have focused on absolute numbers. For example: 7% of vaccinated mothers had babies who contracted pertussis within the first 2 months of life, and 90% of those cases were mild. A comparison of unvaccinated mothers showed the infection rate to be about 43%. That said, this was a retrospective case control study, which can have many “confounders” that could have affected the results (see evidence quality, below).
There is a brief mention of side effects, so this skirts by as Satisfactory. However, it would have been helpful to explain that about 1 in 10 adults experience nausea, vomiting, or chills. Painful swelling in the injected arm that requires medical treatment is “rare,” according to the CDC.
The reporter mentions there is no live bacteria, which is good, but probably should also have mentioned there is no thimerosal, a mercury-based preservative.
The story notes how a mother’s vaccine-created antibodies can transfer to a newborn, but should have thrown out a few more descriptive numbers, including that just 107 cases of pertussis were used in the study. This is a relatively small sample size to draw wide-reaching conclusions, though the story did note these cases were sampled from six different U.S. states.
The type of study was a case control, which is a type of observational study that sits low in the hierarchy of evidence and isn’t capable of proving cause and effect. The story should have made this clear.
Also, it’s worth noting that the story’s headline goes too far in stating that the vaccine “protects” newborns because it’s beyond the scope of this study to prove that any reduction in whooping cough rates was due to the vaccine rather than some other factor.
We didn’t see any unwarranted or scary language.
Included in the story is a quote from vaccine expert Dr. Paul A. Offit, and we detected no potential conflicts of interest.
The alternative would be not getting a Tdap vaccine as a pregnant mother, so we’ll rate this N/A.
It is widely understood that these vaccines are available to almost everyone. The story notes only half of pregnant mothers receive the vaccine, which is important and why the CDC is researching the topic.
The novelty is touched on, though indirectly — this is a relatively new recommendation by the CDC, in 2012, to immunize every pregnant patient during every pregnancy, and this study is an attempt to figure out how protective it is to babies, looking at which trimester the vaccine was given.
There is a quote from a CDC news release, though it is clearly sourced. However, a hyperlink to the release itself would have benefitted readers.
Systematic, Criteria-Driven
The tale of two “negative” studies. Or, how spin is applied to make research findings look more impressive. A must read. https://johnmandrola.substack.com/p/the-tale-of-two-negative-studies
The @nytimes chose to interview one of the editors of the @AnnalsofIM (publisher of this paper) who "gushed with extraordinary glee: It’s huge! There are very few things that reduce your mortality by 30%."
Turns out coffee is not one of those few things despite all that glee. https://twitter.com/garyschwitzer/status/1533202075928215558
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