SAN FRANCISCO (KRON) — The U.S. has three COVID-19 vaccines now on the market, two of which require two doses for maximum efficacy.

According to the Centers for Disease Control and Prevention, over 78 million doses have already been administered nationwide.

But vaccination centers — some with the capacity to inoculate up to 10,000 people per day — are not receiving enough doses to keep up with the appointments.

In San Francisco, the newly-opened mass vaccination site, Moscone Center, had to halt new appointments for nearly two weeks because they ran out of doses, and needed to prioritize second shots.

But now, the vaccine shortage has struck a large healthcare company which is at risk of canceling thousands of second dose appointments.

Sutter Health said it would be forced to cancel 90,000 second dose appointments in Northern California if the state does not send them more vaccines.

The company has already postponed first dose appointments from early February through at least March 9.

This comes even as the U.S. authorized Johnson & Johnson’s one-shot vaccine and gets help from Merck to ramp up the vaccine stock.

President Joe Biden said Tuesday that by May, we would have enough vaccines for every American adult.

But with millions still waiting to get their second dose of the Moderna or Pfizer vaccines, is there an expiration date that precedes that?

According to the latest CDC guidance, the second vaccination is time sensitive:

  • Pfizer-BioNTech vaccine: Get your second shot three weeks (or 21 days) after your first.
  • Moderna vaccine: Get your second shot one month (or 28 days) after your first.

The CDC recommends scheduling the second shot as close to the above intervals as possible. But if necessary, the appointment for either vaccines can be pushed back up to six weeks (42 days) after the first dose.

“COVID-19 vaccines that require two shots may not protect you until a week or two after your second shot,” the CDC advises.

Officials have ‘limited data’ on how effective the mRNA COVID-19 vaccines would be if the second shot is pushed beyond six weeks.

However, the CDC said people do not need to start over with their COVID-19 vaccinations if the second shot is delayed that far out.

The agency recommends scheduling your second appointment with the same provider as your first shot.

People who have only received their first shot should assume they are at risk of getting infected with COVID-19 and still follow social distancing, mask wearing and other safety rules.