Thread by @VikiLovesFACS

How can we know that #COVID19 #vaccines will not harm #fertility? Could an effect unexpectedly appear years into the future?

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Let’s start with the data we have on fertility right now.

👉🏻 No reduction in pregnancy rates in vaccinated (vs unvaccinated) ppl in clinical trials.

👉🏻 Egg quality, fertilisation rates and pregnancy rates are the same in vaccinated and unvaccinated IVF patients. 2/

👉🏻 No difference in sperm quality before vs after vaccination. 3/

If you would like to read these studies (and more!) you can find them here... 4/

drive.google.com/file/d/1_wHIYX…

So we know that vaccination does not harm fertility any time up to about a year immediately after it.

But could some effect suddenly appear years from now, out of nowhere? 7/

To answer this question, we need to take a look at how vaccines work... 8/

The immune response has two phases. The innate phase is fast but non-specific. It triggers the adaptive phase, which is slower but specific.

All effects, whether beneficial (protection) or not (side effects) will come from these phases of the immune response. 9/

The innate phase starts straight away and lasts for a few days. It’s this that gives you the classic annoying side effects of vaccination like a sore arm, temperature or feeling tired. 10/

The adaptive phase kicks in after about 10 days. In the case of COVID vaccination, it protects you against COVID, as illustrated by this now-classic graph from the Pfizer trial. 11/

Any side effects caused by the adaptive phase will also kick in after about 10 days. A good example of this is, for the adenovirus vectored vaccines (AZ and J&J) the rare clotting side effect, which we see 10-24d post-vax. 12/

This is why, in the context of vaccine safety studies, we consider six months to be "long-term". Because it well exceeds the couple-of-months timeframe in which the immune system could do anything unexpected. 13/

Drug development immunologist @andrew_croxford explains more here... 14/

bostonreview.net/science-nature…

This is why, when fertility remains completely normal in the months-to-a-year timeframe following vaccination (which we know to be the case) we have every confidence that that won't change later on. 15/15