Thread by @VikiLovesFACS

Lots of ppl are asking about this article in the Daily Mail. The headline implies we have no idea if the Moderna #COVID19 #vaccine is safe in #pregnancy, despite 1000's having had it.

We actually know that it's safe *because* 1000's have had it... 🧵

dailymail.co.uk/health/article…

Pregnant people were not included in the first rounds of the clinical trials. However, following good safety and efficacy results in the first rounds, both Pfizer and Janssen quickly extended their trials into this group. 2/

Pfizer's trial in pregnant people started in February, and Janssen's in March, both without much fanfare. This is simply Moderna following suit. 3/

But wait! If we don't have clinical trial data on these vaccines in pregnancy, how can I say we know they are safe? 4/

COVID in late pregnancy is associated with a number of poor outcomes, including preterm birth and stillbirth. 5/

bmj.com/content/370/bm…

Meanwhile, from first principles, the vaccines were predicted to be safe in pregnancy.

Because of this, the USA and Israel decided to offer pregnant ppl the vaccines from the outset, and collect their own safety data. 6/

The biggest of these datasets is this one, from the USA. The participants are still being followed up, but the interim report shows no increased risk of miscarriage, preterm birth, stillbirth, SGA or congenital abnormalities following vaccination. 7/

nejm.org/doi/full/10.10…

Here's one from Israel. This focussed on whether vaccination prevents COVID infection in pregnancy (it does!) but as a secondary outcome, they also looked at pregnancy-specific adverse events and found no increased risk in those who were vaccinated. 8/

jamanetwork.com/journals/jama/…

And this one, again from the USA, compared outcomes at birth in those who were vaccinated in pregnancy, versus those who were not. Again, there was no increased risk of adverse outcomes following vaccination. 9/

medrxiv.org/content/10.110…

These three real-world studies actually have rather more pregnant people in them than any of the proposed clinical trials! 10/

So, you might ask, what's the point of the clinical trials then? 11/

First, it's important to note that Pfizer and Janssen started their trials before all this data was available. At that time, there was no guarantee that all this safety data would materialise.

(Though it's good that it did!)

12/

But even so, the trials will tell us some useful things. For example, in the real-world datasets, we have to compare vaccinated people with either historical data, or with those who have chosen to remain unvaccinated. 13/

Neither of these allow us to make comparisons in quite the same way that we can with a group of people who were randomised to not receive the vaccine (and don't know if they did or didn't) as in the trials.

So the trials will give us better comparative data. 14/

Having formal trial data will also be very important for the vaccine manufacturers as they begin the process of applying for full approval for the vaccines (as opposed to emergency approval, which is what they are being used under now). 15/

So... is Moderna trialling their vaccine in pregnant people because we have no idea of how safe it is?

No. The real-world data on this is very reassuring.

But additional data from the trials will give us even more info, as well as being important for full approval. 16/16