Brad Bigford, a traveling nurse practitioner from Boise, Idaho, jumped at the invitation: spend an afternoon at Fred’s Reel Barber Shop in nearby Meridian, offering the flu vaccine to customers.

“Ladies, send your guys for a trim and a flu shot,” Mr. Bigford posted on Facebook. He added, “Anti-vaxxers need not reply.”

Within hours, his Facebook page was swarmed with hundreds of vitriolic comments, even violent threats from people opposed to vaccines. Vicious reviews on Yelp and Google about his urgent-care business, Table Rock Mobile Medicine, popped up from “patients” as far away as Los Angeles, Texas and Australia. Protesters circulated his cellphone number, hometown and wife’s name.

Then the e-cavalry rode to his rescue. A new group of doctors, nurses and other vaccine supporters, called Shots Heard Round the World, flooded his page with evidence-based vaccine facts, which attracted harassers spoiling for a fight to their own sites and away from Mr. Bigford’s. They taught him how to block some 600 posters and expunge comments.