The pandemic is decreasing at the worst critical points. Will last?

LONDON – A month ago, the pandemic looked bleak. More than 750,000 cases of coronavirus have been recorded worldwide in a single day. Infections have increased throughout the United States. New variants identified in the UK, Brazil and South Africa threatened the rest of the world.

But the past month has brought a surprisingly quick, if partial, turnaround. New cases have dropped to half their peak globally, driven largely by steady improvements in some of the same places that have withstood devastating outbreaks this winter.

The cases are an imperfect measure, and uneven records and tests mask the scope of the outbreaks, especially in parts of Africa, Latin America and South Asia. But fewer patients are showing up at hospitals in many countries with the highest rates of infection, giving experts confidence that the decline is real.

“It is a great moment of optimism, but it is also very fragile in many ways,” said Wafaa El-Sadr, an epidemiologist at Columbia University’s Mailman School of Public Health. “We see the light at the end of the tunnel, but it is still a long tunnel.”

The lull in many of the world’s worst outbreaks creates a critical opportunity to keep the virus at bay as vaccinations begin to take effect. Experts believe that vaccines have done little to delay most outbreaks so far, but a small group of countries, especially the wealthy, plan to vaccinate vulnerable groups by spring.

The positive signs come with a series of warnings and risks.

Many countries are still struggling. Brazil is fighting a serious resurgence in the face of a new variant discovered in the country. Hospitalizations in Spain are higher than ever, although official records show a drop in new cases. And in several European countries – the Czech Republic, Estonia and Slovakia – the infection rate is getting worse.

More contagious variants – or just lapses in social distance and other control measures – can still bring new spikes in infections that can overcome the positive effects of vaccination. A variant found for the first time in Britain is spreading rapidly in the United States and has been implicated in outbreaks in Ireland, Portugal and Jordan.

And while most countries saw declines in cases last month, the total global reduction was driven largely by just six countries with huge epidemics.

There is no single cause behind the slowdown, and the factors may differ in different places. Public health experts in the worst-hit countries attribute the progress to some combination of greater adherence to social detachment and wearing masks, the seasonality of the virus and an increase in natural immunity among groups with high rates of existing infection.

Each factor may not be sufficient on its own. Natural immunity, for example, is believed to be well below the levels required to contain the epidemic. But the factors can combine to slow the spread of the virus.

Although the United States has not imposed a national blockade, voluntary changes in behavior, along with some degree of immunity in the hard-hit communities, may have helped prevent an even worse outcome after the holidays, said Caitlin Rivers, an epidemiologist at Johns Hopkins University.

“During the winter, when things were getting really bad, I think people saw how bad things were getting in their community and made different choices,” said Rivers. “They canceled meetings, stayed more at home, took the mask and these things really help, together, to reduce transmission.”

The decline in South Africa had many causes, but the main factor was the intensity of the infection rate last month, said Marc Mendelson, head of infectious diseases and HIV drugs at the University of Cape Town.

“At some point, the virus reaches a barrier, because it cannot find new people to infect and can no longer continue to increase its transmission,” he said.

British experts attribute the decline to a strict national blockade put in place after the holiday season. Vaccines do not explain: although a quarter of the population was vaccinated, only the first recipients had significant protection until January 10, when cases began to decline. These first doses were mostly for health professionals and elderly patients who were already in the hospital.

And some of the worst outbreaks in all of the Americas, southern Africa and Europe peaked during or shortly after the holidays, said Dr. El-Sadr, a researcher at Columbia University. “Over the course of these several months, there were all these occasions for people to mix, mix and travel with family and friends. I think that’s probably also what led to the increase ”.

The challenge of controlling infections until vaccines take effect will be considerably greater in countries with slower vaccination programs.

Vaccination had not yet started in 130 countries earlier this month, according to the World Health Organization, and more than three quarters of the vaccine doses administered were in just 10 countries. Many wealthy nations are accumulating doses, asking for enough to immunize their residents several times, while the poorest nations have yet to receive any.

And a discovery by South Africa that the AstraZeneca vaccine had little effect on a rapidly spreading variant was another blow to countries that planned to have the vaccine relatively cheap and easy to store as part of its launch.

“We are just starting our vaccine campaign in South Africa, and it will be incredibly slow and far from where we would like to be right now,” said Dr. Mendelson. “For countries that have vaccines, it is a slightly different scenario.”

Experts believe that vaccines will play a critical role in keeping infections low, preventing hospitalizations and deaths and even reducing the chance of future mutations if countries are able to vaccinate large ranges of their populations. But the next period will be critical to avoid another wave of infection.

“We have a small window of opportunity here to take advantage of the decreasing number of new infections,” said Bruno Ciancio, head of disease surveillance at the European Center for Disease Prevention and Control. “We must continue with the public health measures in place and vaccinate as many people as possible.”