Johnson & Johnson’s long-awaited vaccine appears to protect against the coronavirus disease 2019 (Covid-19) with just one shot – not as strong as some two-shot rivals but still potentially helpful for a world in dire need of more doses.

J&J said Friday that in the US and seven other countries, the single-shot vaccine was 66-percent effective overall at preventing moderate to severe illness, and much more protective — 85 percent — against the most serious symptoms.

There was some geographic variation. The vaccine worked better in the US — 72-percent effective against moderate to severe Covid-19 – compared to 57% in South Africa, where it was up against an easier-to-spread mutated virus.

“Gambling on one dose was certainly worthwhile,” Dr. Mathai Mammen, global research chief for J&J’s Janssen Pharmaceutical unit, said.

The Philippine unit of Janssen Pharmaceutical announced on Friday the conduct of Phase 3 trials of its Covid-19 vaccine in the Philippines.

With vaccinations off to a rocky start globally, experts had been counting on a one-dose vaccine that would stretch scarce supplies and avoid the logistics nightmare of getting people to return for boosters.

But with some other competing vaccines shown to be 95-percent effective after two doses, at question is whether somewhat less protection is an acceptable tradeoff to get more shots in arms quickly.

“Frankly, simple is beautiful,” said Dr. Matt Hepburn of Operation Warp Speed, the US government’s vaccine initiative.

The company said within a week, it will file an application for emergency use in the US, and then abroad. It expects to supply 100 million doses to the US by June — and a billion doses globally by the end of the year. J&J wouldn’t say exactly how much could be ready to ship as soon as US authorities give the green light.

These are preliminary findings from a study of 44,000 volunteers that isn’t completed yet. Researchers tracked illnesses starting 28 days after vaccination – about the time when, if participants were getting a two-dose variety instead, they would have needed another shot.

After day 28, no one who got vaccinated needed hospitalization or died regardless of whether they were exposed to “regular Covid-19 or these particularly nasty variants,” Mammen said. When the vaccinated did become infected, they had a milder illness.

Early data is mixed on exactly how well all the different kinds work, but shorts made by Pfizer and Moderna appear to be about 95-percent protective after the second dose.

J&J’s Mammen said direct comparisons are difficult because the Pfizer and Moderna vaccines weren’t tested when the pandemic was so severe, with record levels of hospitalizations and deaths plus mutant versions of the virus sweeping some countries.

Dr. Anthony Fauci, the top US infectious disease official, called that a messaging challenge — because the priority is to protect people from hospitalization and death, which J&J’s vaccine appears to do. Especially in places where it’s hard to get people to return for their second dose, the one-dose version could play a key role.

Maybe more important is “a wake-up call” from the study that mutating viruses can challenge vaccines. And the best way to prevent more mutations is “by vaccinating as many people as you possibly can,” Fauci said.

HOW VACCINES WORK

All Covid-19 vaccines train the body to recognize the new coronavirus, usually by spotting the spikey protein that coats it. But they’re made in very different ways.

J&J’s shot uses a cold virus like a Trojan horse to carry the spike gene into the body, where cells make harmless copies of the protein to prime the immune system in case the real virus comes along. It’s the same technology the company used in making a successful Ebola vaccine.

Rival AstraZeneca makes a similar cold virus vaccine that requires two doses. Both the AstraZeneca and J&J vaccines can be stored in a refrigerator, making them easier to ship and to use in developing countries than the frozen kind made by Pfizer and Moderna.

It’s not clear exactly how well the AstraZeneca version, being used in Britain and several other countries, works. Tests in Britain, South Africa and Brazil suggested two doses are about 70-percent effective although there are questions about how much protection older adults get. An ongoing US study may provide more information.