Per Deadline, Penn is fully vaccinated and has taken this stance not out of any concern for himself or his costars, but knowing that unvaccinated crew members not stationed in Zone A can potentially spread the virus.
Beyond Zone A is a far larger assembly of crew members and workers who are not obligated by existing protocols to be vaccinated.
(Zone A includes where unmasked actors congregate and perform on set, surrounded only by key crew members.)
Sean Penn is refusing to return to work on the Watergate-themed Starz limited series Gaslit until the entire production, not just the cast and key crew, has been vaccinated against the COVID-19 virus.