RIO DE JANEIRO — The news received scant notice in the mainstream media, but it quickly gained a foothold in Brazil’s vast right-wing digital landscape: The vaccine on which Brazil had gone all in was a disappointment even to the country that had created it.

“CoronaVac has a low efficacy rate, admits Chinese authority,” crowed one right-wing site.

“The majority of vaccines in Brazil are CoronaVac,” pointed out another.

A top Chinese health official had said the country was considering changes to its vaccines to “solve the problem that the efficacy … is not high.” And in Brazil, in alternative media, a narrative formed: The country was stuck with a second-tier vaccine.

“I always said this vaccine was water with sugar,” one Brazilian commented beneath a story in conservative media.

Now there is fear that the seemingly offhand — and quickly censored — comment by George Gao, the head of the Chinese Center for Disease Control, could further complicate Brazil’s bewildered campaign to vaccinate its population. The very people most likely to get their news from right-wing media are those already more hesitant to get the vaccine.

“We have to be very careful how we communicate science during the pandemic,” said Natália Pasternak, a prominent Brazilian microbiologist. “People already look at this vaccine like ‘second best.’ So this kind of message can have a real impact on vaccine coverage.”

The stakes couldn’t be higher. Brazil is losing around 3,000 people every day to the coronavirus. New variants are storming the country. Attempts to contain the spread have been undone by political divisions, governmental ineptitude, poverty and apathy.

The vaccine has been widely seen as the only way out. But Brazil’s margin of error has been made yet slimmer by the vaccine itself. The efficacy rate of CoronaVac, which accounts for the vast majority of doses administered so far, is barely more than 50 percent. That’s far lower than those being used in the developed world.

“This vaccine of 50 percent, is it good?” Brazilian President Jair Bolsonaro asked when the results were announced earlier this year. Brazilian officials have said the vaccine is 78 percent effective in protecting against moderate and severe covid-19 cases, but 50.4 percent against all cases.

Bolsonaro has been skeptical of all vaccines; he mused at one point that they might turn recipients into crocodiles. But he has directed particular scorn at CoronaVac, which is supported by his bitter political foe, João Doria, the governor of São Paulo state. When trials in Brazil were temporarily suspended after the unrelated death of one of the volunteers, the president celebrated: “One more win for Jair Bolsonaro.”

Vaccine hesitancy is a public health challenge in the best of circumstances. But analysts say it is a greater problem when the vaccine has a lower efficacy rate. Then nearly everyone needs to be vaccinated to control the virus’s circulation.

“I took the vaccine myself and it reduced my chance by 50 percent of getting covid,” said Fernando Reinach, a biochemist at the University of São Paulo. “But with a vaccine that avoids 50 percent of cases, you can’t expect, even if the entire population is vaccinated, that cases will disappear. This is a direct consequence: You’ll keep having cases.”

Chile has vaccinated people more quickly than any other country in the Americas, largely using CoronaVac. But cases there have nonetheless been surging, leading to questions over whether the vaccine is as effective as desired.

A more basic problem, Reinach said, is transparency. The Brazilian research institute Butantan, which has been producing CoronaVac, has provided the government with 40.7 million doses. More than 25 million have been used, according to government statistics. But months into this campaign, it’s still unclear how the vaccine triggers an immune response.

“It’s like having a car that you know runs but not knowing if it runs on gasoline or ethanol,” Reinach said.

Despite the unknowns, CoronaVac appears to have benefited one Brazilian city where nearly everyone has received it. To study the vaccine’s effects, and to glimpse into the future of Brazil’s outbreak, Butantan researchers targeted the southeastern city of Serrana, which had been ravaged by covid-19. By Sunday, all adults had been vaccinated — more than 27,000 people.

In recent weeks, as the vaccine reached more people and statewide restrictions took hold, confirmed monthly cases in Serrana were reduced by more than half. The line of people waiting for ventilators at the hospital vanished. Ten days went by without a single coronavirus hospitalization.

It’s still too early to draw large conclusions, researchers cautioned, but early results appear promising.

“In the last 15 days, we’ve had improvement,” said Glenda Moraes, an epidemiological official in the city. “The vaccine has been positive for the city.”

For Raimundo Ferreira, a 57-year-old residential painter, it has been more than positive. When he received his second dose, he felt like a man blessed. Six of his friends had died of the coronavirus. And even if the vaccine wasn’t as good as others, it was something.

“If there are better vaccines, well, those didn’t come,” he said. “What came was CoronaVac, and we believe in it. We are happy to have it. And we’ve taken it.”

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