The Food and Drug Administration on Saturday issued an emergency authorization for a COVID-19 vaccine developed by Johnson & Johnson, accelerating the nation’s campaign to protect large swaths of the American public as quickly as possible.

The addition of a third vaccine in the U.S. means that more than 100 million people – nearly one-third of the U.S. population – could be vaccinated by the end of March, easing a re-opening of the economy and nudging us closer to long-awaited “herd immunity.”

With shipments planned this week, “the authorization of this vaccine expands the availability of vaccines, the best medical prevention method for COVID-19, to help us in the fight against this pandemic, which has claimed over half a million lives in the United States,” Janet Woodcock, acting commissioner of the FDA, said in a statement.

The shots can’t come fast enough. On Saturday, the U.S. reported a cumulative total of 28.5 million coronavirus cases, representing more than 8% percent of the population.

California is scheduled to receive 380,000 doses of the new vaccine next week and 1.1 million total doses overall the next three weeks, according to California Gov. Gavin Newsom.

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The third vaccine to be approved in the U.S., the new Johnson & Johnson product offers distinct advantages over the existing Pfizer and Moderna vaccines: It requires only one shot and does not need super cold storage, so it is easier to ship and administer.

Based on data provided to the FDA, the Johnson & Johnson vaccine is slightly less effective than the Pfizer and Moderna vaccines. It was 72% effective at preventing all COVID-19 and 86% effective at preventing severe cases of the disease, compared to 95% efficacy reported by the two existing vaccines. Johnson & Johnson is testing a two-dose regimen to see if efficacy increases.

But its ease of use will make it a critical tool for protecting hard-to-reach populations such as rural residents — or anyone who doesn’t want the inconvenience of a second appointment.

Ultimately, the U.S. may consider a policy being proposed by Germany: use more potent vaccines from Pfizer and Moderna in high-risk groups, while vaccinating the general population with a one-dose Johnson & Johnson vaccine or whatever else is available to quickly control the virus’s overall spread.

Consumers are unlikely to be offered a choice of which vaccine to get.  But all three protect against severe illness and death.

“My advice is that you take whichever of these three vaccines you are offered, vaccine scientist Shane Crotty, a professor at the nonprofit La Jolla Institute for Immunology, said on Twitter. “It is WAY better than not being vaccinated!”

In line for vaccines at San Francisco’s Moscone Center on Saturday, Bay Area residents said they had no preferences.

“Any of them, they’re all great,” said Jim Audet, 72, who received a Pfizer inoculation Saturday morning. A former physician and consultant, he is dreaming of travel — wine and good food — once he’s fully vaccinated and overseas opportunities expand.

John Luck, also in line at the Moscone Center, said “people should get whatever they can get, as soon as they can get it.”

Despite a slow rollout of vaccines initially, vaccinations have quickly increased. On January 20, five weeks after distribution began, 8% of Americans over age 65 and 14% of those over 75 had their first shot.  By last week, nearly half of Americans over 65 and 60% of those over 75 have had their first shot.

On Friday, the U.S. administered the most vaccines in a day so far, with nearly 2.4 million shots given.

Initial supplies of the Johnson & Johnson vaccine will be limited, but Johnson & Johnson has promised that 20 million doses will be added to the nation’s distribution pipeline by the end of March.  Also by March, Pfizer promises to have 100 million doses; Moderna, 120 million doses. As two-dose regimens, they will serve a total of 110 million people.

When totaled, those vaccines would protect up to 130 million people by early April.

“I think it’s possible that by April, supply could outstrip demand,” Pfizer board member Dr. Scott Gottlieb, a former Food and Drug Administration commissioner in the Trump administration said on Twitter.

By the end of July, 700 million doses — 300 million from Pfizer, 300 million from Moderna and 100 million from Johnson & Johnson — will have been produced, if manufacturers meet their targets.   That’s enough to protect every eligible American.

But vaccinating the 70% to 90% of people needed to reach “herd immunity” — meaning that enough of the population has either been vaccinated or gained immunity through natural infection to stop viral spread – will be challenging.

Because children under 16 cannot yet be safely immunized, only 80% of the U.S. population is eligible to be immunized. Of those, some are adults who won’t be  vaccinated due to health conditions. And a large percentage – 20% to 30%, depending on the poll – say they either probably or definitely won’t get inoculated.

An additional challenge, Gottlieb said, is to create a vaccine that fends off variants – and deciding whether, and when, manufacturing should switch over to that product.

Moderna has already designed a modified version of its vaccine to protect against variants. It has shipped doses of this vaccine to the National Institutes of Health and is waiting for FDA approval to start a clinical trial.

Experts are also seeking to learn more about the Johnson & Johnson vaccine.  The followup time in the trial was only 60 days, so there’s no sense of how long the one dose will provide protection. The company provided no data about the body’s so-called T cell response, which provides immunologic memory, beyond one month.  And there was no lab data about whether antibodies protect against the worrisome South African variant.

After these vaccines, two other candidates could fill gaps in efficacy, production or distribution.

One is developed by the University of Oxford and AstraZeneca.  The second is made by the Maryland-based start-up Novavax. Internationally, about a dozen vaccines are already being distributed to tens of millions of people. While Russia, China and India have their own vaccines, they have not been widely authorized elsewhere.

At the Moscone Center, San Francisco resident Mary Jane Luck, 69, was thrilled to receive her second dose of the Pfizer vaccine so she could see her elderly father for the first time in nearly a year and travel to Hawaii in June.

No matter the manufacturer, “we were going to go for it,” she said.

Staff writer Fiona Kelliher contributed to this report.