Brazil says CoronaVac has an efficacy rate just over 50 percent, much lower than previously announced. More than 380 million doses have already been ordered.

Scientists in Brazil have downgraded the efficacy of a Chinese coronavirus vaccine that they hailed as a major triumph last week, diminishing hopes for a shot that could be quickly produced and easily distributed to help the developing world.

Officials at the Butantan Institute in São Paulo said on Tuesday that a trial conducted in Brazil showed that the CoronaVac vaccine, made by the Beijing-based company Sinovac, had an efficacy rate just over 50 percent. That rate, slightly above the benchmark that the World Health Organization has said would make a vaccine effective for general use, was far below the 78 percent level announced last week.

The implications could be significant for a vaccine that is crucial to China’s global health diplomacy. At least 10 countries have ordered more than 380 million doses of CoronaVac, though regulatory agencies have yet to fully approve it.

A senior official in Hong Kong, a special administrative region of China that had already ordered CoronaVac, said on Wednesday that an advisory panel would strictly review the vaccine based on clinical trial data before it was rolled out there.

“Those countries that have ordered the Chinese-made vaccines are probably going to question the usefulness of these vaccines,” said Yanzhong Huang, a senior fellow for global health at the Council on Foreign Relations and an expert on health care in China.

“Countries with opposition parties might use this to challenge the decision made by the incumbent government, and that will likely have domestic political implications in these countries,” Mr. Huang said.

Sinovac did not respond to multiple requests for comment.

For months, Chinese officials had said the vaccines made by Sinovac and Sinopharm, a state-owned vaccine maker, would be important tools for fighting the pandemic in poorer countries that do not have extensive health care infrastructures. Unlike the vaccines made by the American drugmakers Pfizer and Moderna, they do not need to be frozen.

The Oxford-AstraZeneca and Johnson & Johnson vaccines, which can be refrigerated and are more effective than their Chinese counterparts, could provide an alternative. But it is now unclear if the governments that have bought CoronaVac can scrap their deals and turn to others.

CoronaVac, unlike some of the other vaccines, relies on older technology that uses chemicals to weaken or kill the virus, which is then put into a vaccine to spark antibodies in the recipient. But the process of killing the virus can weaken a vaccine’s potency, resulting in an immune response that could be shorter or less effective.

The lower efficacy announced Tuesday would mean it would take longer for countries that used CoronaVac’s vaccine to reach “herd immunity,” the point at which enough people are resistant to the virus — roughly 70 percent, many scientists have said — that it is vanquished in a population. By contrast, the vaccines made by Moderna and Pfizer-BioNTech have been shown to have an efficacy rate of about 95 percent.

“This was one of the reasons the Americans and Europeans didn’t go with this older technology,” said John Moore, a vaccine expert at Cornell University. “A well-maintained Ford Model T would probably get you from Wuhan to Beijing, but personally I would prefer a Tesla.”

Brazil’s health regulatory agency, Anvisa, is reviewing data from the trial, which relied on volunteers who are health care professionals. If Anvisa approves emergency use of CoronaVac, officials hope to start giving out shots in Brazil late this month. It has roughly 10.8 million doses of CoronaVac on hand. Last week, Brazil’s health minister, Eduardo Pazuello, said the government intended to buy 100 million doses of CoronaVac.

Natalia Pasternak, a microbiologist and the president of Instituto Questão de Ciência, a Brazilian nongovernmental organization, told reporters on Tuesday that the results of the trial meant it would not be a panacea.

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Answers to Your Vaccine Questions

While the exact order of vaccine recipients may vary by state, most will likely put medical workers and residents of long-term care facilities first. If you want to understand how this decision is getting made, this article will help.

Life will return to normal only when society as a whole gains enough protection against the coronavirus. Once countries authorize a vaccine, they’ll only be able to vaccinate a few percent of their citizens at most in the first couple months. The unvaccinated majority will still remain vulnerable to getting infected. A growing number of coronavirus vaccines are showing robust protection against becoming sick. But it’s also possible for people to spread the virus without even knowing they’re infected because they experience only mild symptoms or none at all. Scientists don’t yet know if the vaccines also block the transmission of the coronavirus. So for the time being, even vaccinated people will need to wear masks, avoid indoor crowds, and so on. Once enough people get vaccinated, it will become very difficult for the coronavirus to find vulnerable people to infect. Depending on how quickly we as a society achieve that goal, life might start approaching something like normal by the fall 2021.

Yes, but not forever. The two vaccines that will potentially get authorized this month clearly protect people from getting sick with Covid-19. But the clinical trials that delivered these results were not designed to determine whether vaccinated people could still spread the coronavirus without developing symptoms. That remains a possibility. We know that people who are naturally infected by the coronavirus can spread it while they’re not experiencing any cough or other symptoms. Researchers will be intensely studying this question as the vaccines roll out. In the meantime, even vaccinated people will need to think of themselves as possible spreaders.

The Pfizer and BioNTech vaccine is delivered as a shot in the arm, like other typical vaccines. The injection won’t be any different from ones you’ve gotten before. Tens of thousands of people have already received the vaccines, and none of them have reported any serious health problems. But some of them have felt short-lived discomfort, including aches and flu-like symptoms that typically last a day. It’s possible that people may need to plan to take a day off work or school after the second shot. While these experiences aren’t pleasant, they are a good sign: they are the result of your own immune system encountering the vaccine and mounting a potent response that will provide long-lasting immunity.

No. The vaccines from Moderna and Pfizer use a genetic molecule to prime the immune system. That molecule, known as mRNA, is eventually destroyed by the body. The mRNA is packaged in an oily bubble that can fuse to a cell, allowing the molecule to slip in. The cell uses the mRNA to make proteins from the coronavirus, which can stimulate the immune system. At any moment, each of our cells may contain hundreds of thousands of mRNA molecules, which they produce in order to make proteins of their own. Once those proteins are made, our cells then shred the mRNA with special enzymes. The mRNA molecules our cells make can only survive a matter of minutes. The mRNA in vaccines is engineered to withstand the cell’s enzymes a bit longer, so that the cells can make extra virus proteins and prompt a stronger immune response. But the mRNA can only last for a few days at most before they are destroyed.

“It is not the best vaccine in the world,” she said at the news conference during which the efficacy rate was disclosed. But she called it a “perfectly acceptable vaccine” that would lead to fewer patients developing serious cases or dying from the virus.

Dimas Covas, the director of the Butantan Institute, called the vaccine an “excellent” tool “waiting to be used in a country where currently 1,000 people are dying per day.”

Many of the countries that have ordered CoronaVac are relatively poor, desperate to halt the pandemic and protect their populations.

Indonesia, for example, has ordered 125.5 million CoronaVac doses. The country has reported nearly 850,000 coronavirus cases and nearly 25,000 deaths, the highest numbers in Southeast Asia.

On Wednesday morning, its president, Joko Widodo, was injected with CoronaVac on live television, kicking off a national vaccination program. “Covid vaccination is important for us to break the chain of transmission of this coronavirus and provide health protection for all of us,” Mr. Joko said after getting his shot.

Sulfikar Amir, an Indonesian associate professor of disaster sociology at Nanyang Technological University in Singapore, said the latest results from Brazil were a cause for concern.

“Why doesn’t Indonesia wait for a better vaccine?” he asked. “My impression is that this is rushed and forced.”

In the short term, some countries may have few viable options. Governments, many of them in richer nations, have already locked in more than half the doses that could come onto the market by the end of next year.

To its comparative advantage, Sinovac has immense manufacturing capacity. The company has said it can make 600 million doses this year.

In China, the weaker efficacy data from Brazil could be a setback for the country’s biotech ambitions. It had pinned its hopes on making a Covid-19 vaccine that would burnish its credentials as a global scientific power.

The tepid results could also be problematic for Chinese officials, given that they had touted the efficacy of the vaccines made by Sinovac and Sinopharm. Even though the vaccines had not received regulatory approval, and data from late-stage trials had not been made public, Beijing gave them to thousands of Chinese people under an emergency use policy; it plans to vaccinate 50 million people by the middle of next month.

State media in China played down the news from Brazil. Global Times, a state-owned nationalist tabloid, ran a headline that said the Sinovac vaccine was “100 percent effective in preventing severe cases, could reduce hospitalizations by 80 percent.”

The new data could heighten skepticism among people around the world who are already wary of Chinese-made vaccines, given that the country has a history of vaccine quality scandals. A study from the Chinese University of Hong Kong found that just 37.2 percent of respondents in Hong Kong were willing to be vaccinated.

Scientists had already raised questions about the piecemeal way in which efficacy data about the Chinese vaccines had been released. Indonesia said on Monday that its interim analysis found CoronaVac to have an efficacy rate of 65.3 percent. Last month, Turkey said it had an efficacy rate of 91.25 percent, but that was based on preliminary results from a small clinical trial.

The vaccine had long taken on a political dimension in Brazil. President Jair Bolsonaro had spoken derisively about CoronaVac, fueling an growing anti-vaccination movement in the country, where more than 200,000 people have died from Covid-19. The vaccine had been championed by São Paulo’s governor, João Doria, who is widely expected to run for president in 2022 and is among Mr. Bolsonaro’s most vocal critics.

In Brazil, officials say the higher efficacy rate previously announced for CoronaVac pertained to the protection it offered against developing Covid-19 symptoms significant enough to require treatment. While officials had asserted last week that the vaccine provided absolute protection against moderate to severe symptoms, they had not disclosed another group who had “very mild” infections despite having been vaccinated.

Denise Garrett, a Brazilian-American epidemiologist and vaccine expert, said there was no reason to doubt CoronaVac’s safety, adding that the data presented so far suggested it would provide a satisfactory level of protection. But Dr. Garrett said the vague and sometimes misleading manner in which information about the vaccine had been made public could shake people’s confidence in its reliability and fuel the political battle over the vaccine.

“The lack of transparency really damages people’s trust,” she said. “They’ve just reinforced the narrative that this vaccine is not good.”

Reporting was contributed by Manuela Andreoni, Letícia Casado, Richard C. Paddock and Muktita Suhartono. Elsie Chen contributed research.

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