California tightened its pandemic rules for large events on Wednesday, as officials continued to plead with holdouts to get fully vaccinated.

At indoor gatherings of 1,000 people or more, attendees will have to present proof of vaccination or a negative COVID test within 72 hours.

The requirement previously applied just to venues where 5,000 or more people gathered indoors, and self-attestation was allowed in lieu of actual proof of vaccination.

San Francisco already has stricter rules in place — last week, the city announced that all indoor venues with 1,000 or more attendees would be subject to its vaccine requirement (children under 12 being an exception), and testing out is not an option.

But the change will affect other large venues around California, including sports arenas and concert halls.

Meanwhile in San Francisco, Mayor London Breed made a plea for holdouts to get their shots. Seventy-nine percent of eligible San Franciscans are fully vaccinated.

“I am getting calls on a regular basis hearing about friends who have passed away because they have contracted COVID,” Breed said on Wednesday, adding that “none of those people were vaccinated.”

The city — whose vaccine mandate takes effect starting Friday — is experiencing its third-highest number of new daily cases since the start of the pandemic, with 115 residents hospitalized with COVID-19. Breed said all of those who are not yet vaccinated are at high risk: “It’s not a matter of if you will catch the virus. It’s a matter of when.”

San Francisco officials also unveiled plans on Wednesday to offer a third dose of the Pfizer-BioNTech or Moderna coronavirus vaccine to certain immunocompromised patients, including those who have had organ transplants, received treatment for blood cancers or are in the advanced stage of an HIV infection.

Others covered include those who have received a stem cell transplant within two years and are currently on immunosuppressive medications, those with “moderate or severe immunodeficiency” and those who are taking high doses of corticosteroids or other immunosuppressive drugs. All told, the number of eligible people is expected to be fairly small, according to the health department.

“For now, we ask that only individuals who meet the criteria for immune compromise request a third dose,” Dr. Grant Colfax, the city’s health director, said in a statement.

In order to receive a third dose of an mRNA vaccine, patients should first talk with their health care provider to see whether it’s “appropriate and safe for them,” he said — and if possible get their doses through that provider.

Marin County announced similar steps Wednesday for 10,000 immunocompromised residents. Now those patients will be eligible for a third dose of mRNA vaccine 28 days or more after their second dose.

On a national level, the Biden administration announced it would recommend booster shots of the COVID vaccines eight months after people get their second shot of the Pfizer or Moderna vaccine. The booster doses could begin the week of Sept. 20.

Staff writers Aidin Vaziri, Rita Beamish and Ryan Kost contributed to this report.

Kate Galbraith is a San Francisco Chronicle staff writer. Email: [email protected] Twitter: @kategalbraith