Blood samples from people who received three Pfizer doses showed robust antibody levels against Omicron.

Pfizer and BioNTech said Wednesday that laboratory tests suggest that three doses of their coronavirus vaccine offer significant protection against the fast-spreading Omicron variant of the virus.

The companies said that tests of blood from people who received only two doses found much lower antibody levels against the Omicron variant compared with an earlier version of the virus. That finding indicates that two doses alone “may not be sufficient to protect against infection” by the new variant, the companies said.

But the blood samples obtained from people one month after they...