TEL AVIV — Israel's fast-moving vaccination program, which has already reached a third of the population, is built on a bargain. The vaccine maker agreed to keep up the quick-time delivery of doses in exchange for partial access to the vast database of information maintained by the country's national health-care system.

Already, researchers here and in other countries are getting their first look at how the crash-effort vaccine performs at population level. The Israeli results show cause for hope, including preliminary findings that infection rates plummeted to near zero in 128,000 people who got two rounds of vaccine. Other findings, though, revealed less immunity than expected after just one dose, leading health officials in some countries to retool their rollout efforts.

But some privacy advocates are complaining that Israelis are being used as guinea pigs without their permission after learning that Pfizer, maker of the vaccine that Israel has injected into the arms of about one third of its 9 million people, got access to health data as a condition of the deal.

Former deputy attorney general Yoram Bar-Sela cautioned in an op-ed for the newspaper Haaretz that “it is doubtful that either the state or the HMOs have any authority at all to transfer the details of the vaccination recipient to any foreign body,” even in the case of a pandemic. And advocates are warning that a data breach could expose sensitive information, even if Pfizer had planned to keep it confidential.

The government says, however, that Pfizer will see only anonymous aggregated results, with vaccine recipients grouped by age and other demographic criteria, and that no confidential information will be revealed. “Sharing personal health information is a crime,” Israeli Health Minister Yuli Edelstein said in a radio interview Wednesday.

As countries compete to get vaccine doses, anger is rising in Europe that doses are flowing faster to some other parts of the world. The European Union has threatened legal action after AstraZeneca and Pfizer announced delivery delays.

Israel, which some European officials have noted paid a premium for its doses and gets its supply from a Pfizer factory in Belgium, says it remains on track to expand vaccine eligibility in the coming weeks to everyone age 30 and up.

Israel’s Pfizer contract was made public in early January but with the specifics of cost and data sharing redacted. According information obtained by the Israeli media, Israel paid about $50 per vaccine, roughly double the cost to European Union countries buying as a bloc.

“Presumably we paid more than the E.U., but we paid according to market value,” Edelstein said in an interview. “When Europeans were negotiating a deal for half a billion doses, we were negotiating for several million doses.”

Pfizer has privately committed to shipping about 10 million doses to Israel by the end of March, according to Israeli media accounts, a tiny portion of the 1.2 billion doses the company has pledged to countries across the world in 2021. Pfizer is backing Israel’s aggressive push because early measurable success here would boost not only the fight against covid-19, but also the company’s bottom line, according to Amnon Lahad, chairman of the National Committee for Community Healthcare, which advises the Ministry of Health.

“If we show that we are able to control the disease and that Israel will be able to go to near-normal life faster than any other country, that will be a marvelous commercial for Pfizer to sell all over the world,” Lahad said.

Pfizer did not respond to a request for comment on its contract with Israel.

Israel was among the first recipients of the Pfizer vaccine in early December. And the flow has continued steadily, with Pfizer flying in 400,000 to 700,000 doses every week, according to Israeli media.

Edelstein said Israel is considered a good test case because of its meticulously digitized, decades-spanning trove of data, gathered from a mandatory universal health-care system involving four not-for-profit HMOs.

As a country of 9 million with a relatively small elderly population, Israel has inoculated most older residents and begun vaccinating the wider public. The government is preparing “green passports” for those who have received both doses, which would exempt them from quarantine and eventually grant them access to public places like theaters and restaurants.

The Ministry of Health has now opened vaccinations for citizens over the age of 35 and for those 16 to 18, a group that has recently exhibited soaring infection rates and that in coming months will prepare for high school matriculation exams.

Health-care administrators announced Monday that the vaccine may be even more effective than the 95 percent level found during trials.

Maccabi, one of Israel’s four HMOs, reported preliminary findings that just 0.015 percent of people became infected with the coronavirus in the week after receiving their second shot. Among the positive cases, none exhibited severe symptoms.

Another study, out of the Sheba Medical Center near Tel Aviv, showed the vaccine to be 98 percent effective among 102 medical workers who had received both shots and suggested that recipients of the double dose are unlikely to become carriers of the virus. “There is definitely reason for optimism,” said Gili Regev-Yohai, director of the Infection Prevention and Control Unit at Sheba Medical Center and editor of the survey.

But Israel’s Weizmann Institute of Science found that a single dose of Pfizer’s vaccine was significantly less effective than had been indicated by the company’s clinical study.

In response to this preliminary result, Gaston De Serres, an adviser to Quebec’s vaccine program, said it may scrap plans to release a larger number of first doses and prioritize delivering the second shot within 21 days. President Biden has also floated the idea of allocating more doses to first-time vaccination supplies while reducing the reserve for second shots.

As the number of severe coronavirus cases begins to level off, epidemiologists said Israel might be able to achieve 80 percent immunity among its highest-risk groups by February and 95 percent of that population by March. That could begin to allow a return to pre-pandemic life.

Speaking to the Davos World Economic Forum on Wednesday, Prime Minister Benjamin Netanyahu said Israel could be a “world laboratory for herd immunity.”

Even as Israelis are starting to see relief from the pandemic, many want to know just what private information they are now obliged to share with one of the world’s largest pharmaceutical corporations. A petition by a government watchdog group, the Movement for Quality Government in Israel, has appealed for the full details of the Pfizer deal to be made public.

The Pfizer contract comes after privacy concerns spiked in Israel last year when the government allowed the Shin Bet, the country’s internal security agency, to track citizens’ cellphones and notify those who had been close to an infected person. The program sparked lawsuits and outrage from people who said they were wrongly forced to quarantine. Despite limited effectiveness, the program has received repeated government authorization.

Privacy worries aside, many Israelis seem to view a deal that hastens a return to normalcy as worth it, especially as they trudge through their third national lockdown and confront a surge of more than 8,000 new confirmed cases each day.

“Just so happened, we had the privilege of being chosen as the first country to do this experiment,” said Eldad Sibton, a Tel Aviv restaurateur. “When anti-vaccine citizens of other countries will look at us and say, all right, Israel has vaccinated and then started to have parties, to go back to life, they’ll want in, too. This will have a positive impact for all of humanity.”

Loveday Morris in Berlin contributed to this report.