A month ago, the pandemic looked bleak. More than 750,000 coronavirus cases were tallied worldwide in a single day. Infections surged across the entire United States. New variants identified in the United Kingdom, Brazil and South Africa threatened the rest of the world.

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

Cases are an imperfect measure, and uneven records and testing mask the scope of 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’s a great moment of optimism, but it’s also very fragile in a lot of 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’s still a long tunnel.” The lull in many of the world’s worst outbreaks creates a critical opportunity to keep the virus in retreat as vaccinations begin to take effect. Experts believe vaccines have done little to slow most outbreaks so far, but a small group of countries, primarily wealthy ones, plan to vaccinate vulnerable groups by the spring.

The positive signs come with a number of caveats and risks.

Many countries are still struggling. Brazil is battling a serious resurgence in the face of a new variant discovered in the country. Hospitalisations in Spain are higher than they’ve ever been, even though official tallies show a decline in new cases. And in a number of European countries — the Czech Republic, Estonia and Slovakia — the infection rate is worsening.

More contagious variants — or just lapses in social distancing and other control measures — could still bring new spikes in infections that could outpace the positive effects of vaccination. A variant first found in Britain is spreading rapidly in the United States, and it has been implicated in surges in Ireland, Portugal and Jordan. And while most countries have seen declines in cases over the past month, the total global reduction has been driven largely by just six countries with enormous epidemics.

There is no single cause behind the slowdowns, and the factors may differ in different places. Public health experts in the worst-hit countries attribute the progress to some combination of increased adherence to social distancing and mask wearing, the seasonality of the virus and a build-up of natural immunity among groups with high rates of existing infection. Each factor may not be enough on its own. Natural immunity, for instance, is believed to be well short of levels required to stop the epidemic.

But the factors can combine to slow the rate at which the virus spreads. Although the United States did not impose a national lockdown, voluntary changes in behaviour, along with some degree of immunity in 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,” Dr. Rivers said. “They cancelled gatherings, they stayed home more, they reached for the mask, and those things really do help, put together, to reduce transmission.”

The decline in South Africa has had many causes, but the main driver was the sheer intensity of the infection rate last month, said Marc Mendelson, the head of infectious disease and H.I.V. medicine at the University of Cape Town.

“At some point the virus hits a barrier, because it cannot find new people to infect, and it can no longer continue increasing its transmission,” he said. British experts attribute the decline to a strict national lockdown put in place after the holiday season. Vaccines don’t explain it: Even though a quarter of the population has been vaccinated, only the earliest recipients had significant protection by Jan. 10, when cases there started to drop. Those early doses mostly went to health-care workers and elderly patients already in the hospital.

And some of the worst outbreaks all over the Americas, southern Africa and Europe peaked during or just after the holidays, said Dr. El-Sadr, the Columbia University researcher. “Over these several months there have been all these occasions for people to mingle, mix and travel with family and friends. I think that also was probably driving that surge.”

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

Vaccinations had not begun at all in 130 countries as of early this month, according to the World Health Organization, and more than three-quarters of the vaccine doses administered have been in just 10 countries. Many rich nations are hoarding doses, ordering enough to immunise their residents multiple times over, while poorer nations have yet to receive any.

And a finding from South Africa that the AstraZeneca vaccine had little effect on a fast-spreading variant dealt another blow to countries that had planned to rely on the relatively cheap, easy-to-store vaccine as a part of their rollout. “We are just starting our vaccine campaign in South Africa, and it’s going to be incredibly slow and nowhere near where we wanted to be right now,” Dr. Mendelson said. “For countries that have vaccines, it’s a slightly different landscape.”

Experts believe that vaccines will play a critical role in keeping infections down, preventing hospitalisations and deaths and even reducing the chance of future mutations if countries are able to vaccinate large swaths 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, the 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.”

The writers are journalists with NYT