RIO DE JANEIRO — Brazil currently accounts for one-quarter of the entire world’s daily COVID-19 deaths, far more than any other single nation, and health experts are warning that the nation is on the verge of even greater calamity.

The nation’s seven-day average of 2,400 deaths stands to reach to 3,000 within weeks, six experts told the Associated Press. That’s nearly the worst level seen by the U.S., though Brazil has two-thirds its population. Spikes of daily deaths could soon hit 4,000; on Friday there were 3,650.

Having glimpsed the abyss, there is growing recognition shutdowns are no longer avoidable – not just among experts, but also many mayors and governors. Restrictions on activity they implemented last year were half-hearted and consistently sabotaged by President Jair Bolsonaro, who sought to stave off economic doom. He remains unconvinced of any need for clampdown, which leaves local leaders pursuing a patchwork of measures to prevent the death toll from spiraling further.

It may be too late, with a more contagious variant rampaging across Brazil. For the first time, new daily cases topped 100,000 on March 25, with many more uncounted. Miguel Nicolelis, professor of Neurobiology at Duke University who advised several Brazilian governors and mayors on pandemic control, anticipates the total death toll reaching 500,000 by July and exceeding that of the U.S. by year-end.

“We have surpassed levels never imagined for a country with a public health care system, a history of efficient immunization campaigns and health workers who are second to none in the world,” Nicolelis said. “The next stage is the health system collapse.”

The system is already buckling, with almost all states’ intensive care units near or at capacity. Dr. José Antônio Curiati, a supervisor at Sao Paulo’s Hospital das Clinicas, the biggest hospital complex in Latin America, said its beds are full, but patients keep arriving. The city’s oxygen supply isn’t guaranteed, and stocks of sedatives required for intubation in intensive care units will soon run out.

“Four thousand deaths a day seems to be right around the corner,” Curiati said.

On March 17 in northeastern Piaui state, nurse Polyena Silveira wept beside a COVID-19 patient who died on the floor for lack of beds at her public hospital. A photo capturing the moment went viral and served as a national wake-up call.

“When he was gone, I had two minutes to feel sorry before moving to the next patient,” Silveira, 33, told the AP. “In eight years as a nurse, I’d never felt as much pain as that night. I’m near my limit, physically and mentally.”

Brazil’s state-run science and technology institute, Fiocruz, on Tuesday called for a 14-day lockdown to reduce transmission by 40 percent. Natalia Pasternak, a microbiologist who presides over the Question of Science Institute, pointed to a local example of success: The mid-size city of Araraquara in Sao Paulo state last month implemented lockdown and has seen its cases and deaths recede.

Pasternak declined to estimate Brazil’s looming daily death toll but said the trend is for continued growth if nothing is done.

“We need coordinated action, and that’s probably not going to happen because the federal government has no real interest in pursuing preventative actions,” Pasternak said. “(Mayors and governors) are trying to implement preventative measures, but separately and in their own ways. This isn’t the best approach, but it’s better than nothing.”

The World Health Organization’s director, Tedros Adhanom Ghebreyesus, on Monday called for everyone in Brazil to muster a serious response – “whether it’s the government or the people.”

“It’s a concerted effort of all actors that will really reverse this upward trend. It is actually very fast and accelerating really, really fast,” he said. “Especially we’re worried about the (weekly) death rate, which doubled in just one month from 7,000 to 15,000.”

The virus’s spread has been turbocharged by the more contagious P.1 variant that has become cause for concern beyond Brazil’s borders, not just in South America. It has already been identified in the U.S., last week in New York and Maine. Dr. Anthony Fauci, the top U.S. infectious disease expert, said Wednesday that his team will be meeting with Brazilian authorities and are “quite concerned” about the situation in Brazil.

The U.S. has seen its death toll plunge since late January amid a massive vaccine rollout, and its seven-day average has dipped below 1,000. By contrast, Brazil’s vaccine rollout has been strained, at best. The government bet big on a single vaccine provider, AstraZeneca, while for months rejecting offers to purchase others. Only after delivery delays from AstraZeneca jeopardized rollout did Brazil’s health ministry begin buying — but too late for most deliveries to arrive in the first half of this year.

The nation has fully vaccinated less than 2 percent of its citizens, which experts widely consider an embarrassment for a country long regarded as a global model for vaccination programs.