UK Vaccines & Pregnancy- Part 3, Still Concerned

Updating a key data source and re-calculating with possible scenarios

Earlier this week I wrote two posts regarding the data on vaccinations and pregnancy in the UK reports.

In the 1st post, I noted the following:

• Most (71.5%) women who were vaccinated during pregnancy were vaccinated in the 3rd trimester (source: UK Weekly Vaccine Surveillance Report )

• Roughly half (47%) of fetal deaths after week 20 (to my knowledge, this is synonymous with stillborn death) occur before the 3rd trimester (source: US CDC Report)

• From this I estimated that vaccinated women should have stillbirth rates ~33% lower than unvaccinated women, while the UK data is showing they experienced stillbirths ~7.5% lower.

I noted in the 1st post that a limitation of my analysis is that it did not account for age of the women. Namely, older women got vaxxed at a higher rate and older women have stillbirths at a higher rate. The 2nd post looked to estimate what effect age might have on the stillbirth rates. The UK Report had information on age distribution of vaccine recipients while the CDC had information on the relative risk of stillbirths for different ages. Combining the two, I estimated that the increased risk of stillbirths for vaccinated women (based on being older) was ~4%. This serves to mitigate the numbers estimated in the 1st post by 4%.

Thanks to comments from my readers, I discovered that the UK provides a rich data set on stillbirths. In looking at table 8 of this table, I found that the UK data indicates that only 30% of your risk of stillbirth has gone away by the start of the 3rd trimester (week 27), rather than 47% used from the CDC reference. Here is the table:

Ugh! If only 30% of the risk is actually passed by the 3rd trimester, then my estimate of the potential increased risk of stillbirths for vaccinated women was too high! The last thing I wanted to do was alarm or mislead on such a sensitive topic.

Sidebar- I believe the reason the UK and US numbers differ significantly here is that it would appear the UK defines a stillbirth as a fetal death only starting in week 24, while the US data included deaths starting in week 20 of pregnancy.

Then I realized that the granular data from the UK (stillbirths by week) would allow me to make a better estimate than before. Previously, my estimate assumed all women vaccinated in the 3rd trimester faced a stillbirth risk equal to the remaining risk of stillbirth at the start of the 3rd trimester. This is equivalent to assuming all the 3rd trimester vaccinations happened in the 1st week (week 27) of the 3rd trimester. This is obviously implausible, but provides the most conservative estimate. Since we don’t know the exact timing of the vaccinations in these women, let’s explore 3 scenarios:

1. Assume all 3rd trimester vaccinations happen at the start of the 3rd trimester (week 27). This will be the most conservative estimate in terms of calculating potential increased risk of stillbirths from vaccination.

2. Assume all 3rd trimester vaccinations happen at the end of the 3rd trimester (week 39). This would result in the most aggressive estimate of potential increased risk.

3. Assume vaccinations are evenly distributed across the 3rd trimester (between weeks 27-39).

Now we can estimate what we believe the rate of stillbirths might be (relative to unvaccinated women) based on 28.5% being vaccinated before the 3rd trimester (assume they face full risk of stillbirth) and the 3 scenarios above in terms of timing of vaccination within the 3rd trimester:

Recall that the UK report shows vaccinated women experienced stillbirths at a rate of 93% of that of unvaccinated women.

• In our estimate, at the most conservative end where all 3rd trimester vaccinations happened in week 27, they should have experienced stillbirths at a rate of 82% of the unvaccinated, indicating an increased risk of 11% from the vaccine (lower bound estimate).

• At the other end, if all 3rd trimester vaccinations happened in week 39, then stillbirths in the vaccinated should have occured at a rate of 36% of the unvaccinated, indicating an increased stillbirth rate of 57% (***I don’t believe this to be a realistic scenario, but it is useful as an upper bound).

• Finally, in perhaps the most realistic scenario, if women got vaccinated throughout the 3rd trimester evenly, then they should have had miscarriages at a rate of 60% of the unvaccinated women, meaning the vaccination increased their risk of stillbirth by 33%.

I am relieved that the numbers presented in previous posts were not wildly overstated (in fact, might have been understated). Would be great if we had more detailed data on timing of vaccination in pregnancy so instead of exploring these scenarios we could perform the direct calculation.