50% is normal in the first trimester.

It could be that the vaccine is boosting the numbers that make it past the first three weeks, which would also cause a higher number to appear.

For every one pregnancy, typially four are lost. Typically only one of those is noticed, hence 50/50.

The vast majority of miscarriages are from chromosomal anomalities not camptible with life.

Let's say it's the vaccine. What does 75% mean really.

Well, the true rate before was 80% but only one made it be noticed so it was 1 out of 2 to make 50%.

Let's work that backwards. For 75% there are roughly four being noticed and only one making it.

Assuming the vaccine is NOT helping more pregnancies reach later in the first trimester, then 1 out of 4 noticeable pregnancies are making it.

Simple terms or TL;DR: The vacinne DOUBLES the rate of miscarriage, mean it will HALVE the birth rate.