1) Vaccines are
not 100% safe.
Please visit Vaccine Adverse Event Reporting System:
VAERS Data for information about adverse effects of vaccines. Note, this is a US government sponsored website run by Health and Human Services,
not some conspiracy website. It is a collection site and not all reports are corroborated. Nevertheless, there are more than 45,000 reported adverse reactions in 2016 alone, including death. That is a typical year. Even if half of them are legitimate, that is many incidents. Even 10%.
Immunizations are a calculated risk. The risks in general are greater for not getting immunized, but any medical person telling you they are perfectly safe is not being honest.
2) Vaccines are not 100% effective.
I teach statistics and vaccine efficacy is one subject we study. One reason you receive multiple immunizations for diseases such as polio is that it is only 95% effective. You can be immunized and
still contract the disease as we saw with the measles outbreak at Disneyland a few years ago. The risk is compounded by the size of the non-immunized population.
In summary, vaccines are a poison pill agreement among an informed population that will negatively affect one in one million versus one in one thousand without the vaccine. It is worth the risk, but there
is a risk. Both my children are fully immunized and I get a flu vaccine every year. I gave it a lot of thought and don't harbor any ill regards towards parents who choose otherwise.