Pinterest has stopped returning results for searches related to vaccinations, a drastic step the social-media company said is aimed at curbing the spread of misinformation but one that demonstrates the power of tech companies to censor discussion of hot-button issues.

Most shared images on Pinterest relating to vaccination cautioned against it, contradicting established medical guidelines and research showing that vaccines are safe, Pinterest said. The image-searching platform tried to remove the antivaccination content, a...