2shoes not necessarily
herd immunity requires a base level of children to be vaccinated, not necessarily all- for examle if all the children in your dc's classes were vaqccinated then your child is protected as they can't pass it on. So if only children at risk of ASD were not vaccinated herd immunity could still be maintained (there's a lot of dispute about herd immunity and percentages vary but most stuff I read suggests about 80% vaccinated children leads to herd immunity).
Herd immunity itself is a disputed conceopt on MN and there's lots of other stuff- for example in the most recent outbreak of measles here the majority of children (vast mjority( had been vaccinated- so pushing illnesses such as mumps and rubella into more worrying age ranges (adolecscents / ypung poeple- mumps damage in males and rubella in pregnant females). There's a wide thought that MMR protection doesn't last as long as people think.
And even if you do think vaccination is essential- there's always the single jabs as we have chosen (as we do believe in vaccination). The trouble is theya re only available privately so, especially in the current climate, people will increasingly be forced into none rather than singles due to cost.
As for the ASD thing- no, nobody knows for sure but the idea is that ASD is often (not always- lots to be learned) a genetic thing that requires a 'trigger' to start it; MMR may be one of those triggers for a few children.