It gives me no pleasure at all to insult a Torah sage but I'm not going to sugarcoat it: Rabbi Shmuel Kanievsky is an anti vaxx moron* and the letter he signed ordering Lakewood schools to admit unvaccinated kids puts children in mortal danger.
This is not the time or the place to rehash the facts of herd immunity or to explain again how science works. Anyone truly interested in the emes can consult wikipedia. Instead I want to urge you to tell your schools and rabbis to stop playing fast and loose with the health of our children. If a blue shirt or a short skirt or a TV set puts children at terrible risk what does the measles or whooping cough virus do? If a striped shirt or a blue pair of shoes are banned from entering our schools why are we opening the door for deadly pathogens?
*This doesn't detract from his formidable torah knowledge or his sterling character. You can simultaneously be a wonderful person a torah giant and a flaming ignoramus regarding natural sciences.