“There appear to be millions of Americans walking around who have received a single dose of a Covid-19 vaccine, who may think they are protected against whatever the virus can throw at them — and who could be sorely wrong.
“I’m not sure we should regard them as equivalent to unvaccinated people,” Angela Rasmussen, a virologist at the Vaccine and Infectious Disease Organization, told me. “But they are at higher risk than fully vaccinated and boosted people.”
That was the early consensus among the experts I consulted, and the preliminary data shows, as expected, low effectiveness against omicron after one dose of the Pfizer/BioNTech vaccine. The effectiveness against omicron also declines over time after two doses but is restored to high levels (76 percent efficacy against infection) after a third dose. This was a fairly small study out of the UK, and more data will be forthcoming, but it gives an initial picture of how the vaccines are holding up against the new variant.
People who have received only one dose of a vaccine could conceivably be almost as vulnerable to infection from omicron as the unvaccinated. They may still have some level of protection against severe illness because of the multiple layers of immunity induced by the vaccines. But it’s an open question at this point — and may soon become an urgent one for the Americans who fall into this camp.”
https://www.vox.com/coronavirus-covid19/22826745/covid-19-pfizer-moderna-vaccine-omicron-variant