In a few weeks, an estimated 20,000 ministers, activists and executives from nearly every country in the world are set to descend on Glasgow, Scotland, for a summit to hammer out how to make progress on climate change.

And yet, the Marshall Islands chief negotiator still doesn’t know how many people from her country are coming with her. An activist from Kenya has no idea when, or if, he will get vaccinated against COVID-19, while another from Mexico has flown to the United States to get a dose. And the British government hosts are still trying to figure out how to get Scotland’s health labs ready to process coronavirus tests in case of an outbreak.

The climate summit, known as the 26th session of the Conference of Parties, or COP26, will be one of the biggest international gatherings held during the COVID-19 pandemic when it starts Oct. 31. Among those expected to attend are Queen Elizabeth, Pope Francis and at least 100 presidents and prime ministers, including President Joe Biden of the United States.

The stakes are exceptionally high.

The biggest polluting countries in the world are under the spotlight to show whether they can do what is necessary to stave off the worst effects of climate change. Coronavirus cases continue to soar. And many of the summit participants will be coming from countries where vaccines are still not widely available, particularly the global south. Globally, fewer than half of all adults have been vaccinated against COVID-19, illustrating the inequities of vaccination.

“Organizing a COP is a huge, huge challenge anyway,” said Alok Sharma, a veteran British politician in charge of this conference, in a recent interview in Washington, D.C. “Organizing a COP in COVID, the challenge has been magnified.”

Despite some calls to hold the conference virtually or postpone it — as last year’s gathering was — Sharma has been adamant that leaders need to gather in person to address the climate crisis. He pledged that Britain will try to run the mass gathering in a way that minimizes the likelihood of infections. But risks abound.

Conference organizers have said vaccinations are encouraged but the United Nations, under whose auspices the annual climate negotiations are held, does not require them at its meetings. There’s also no way to verify whether vaccine certificates are legitimate. Britain has offered free vaccines to anyone who wants them, though many say they have yet to receive them.

Alex Saier, the spokesperson for the United Nations climate agency, said by email that his office worked with the British government and the World Health Organization to develop health protocols.

“The collective decision was to strongly encourage all participants to be vaccinated before coming to the COP for the health and safety of all, but to not make it mandatory since some participants have medical or other issues that preclude them from vaccinations,” Saier said.

In any case, presidents and prime ministers can’t be required to quarantine, because of diplomatic immunity provisions. And so Sharma’s team is counting on good behavior.

Delegates will have to sign a code of conduct, stipulating that they will follow public health protocols, including daily coronavirus testing to enter the main venue and wearing masks while walking the hallways. Negotiators who normally huddle for hours in windowless rooms, debating commas and verbs in official documents, will also be encouraged to keep their masks on.

Leaders from environmental groups and other nongovernment organizations who typically monitor the negotiations will have limited access to rooms where those sessions are held. Every nation is being told to trim the size of its delegation.

“We want to ensure a safe event,” Sharma said.

About 1,000 people have requested vaccines and Sharma said “several hundred” have been vaccinated through the British government’s program, though his office would not say precisely how many. Britain is encouraging delegates to receive vaccines administered by their own national programs but that hasn’t been possible for some attendees .

Consider the case of Nobert Nyandire of Kenya. When Britain offered vaccinations, he applied.

That was in July. He is still waiting.

Nyandire is with the East Africa chapter of Climate Action Network, which represents more than 1,000 nongovernmental organizations.

The United Nations told him in early September that vaccinations, provided by Britain, would begin soon in his country. Three weeks later, the United Nations suggested that he rely on Kenya’s national vaccination program. He says that communications have been confusing and that he still hopes to get the vaccine, though there is no guarantee.

A fellow activist in Mexico, Maria Reyes, worried that the vaccine offer from Britain wouldn’t come in time. She flew to Los Angeles, got a Johnson & Johnson dose at the airport, and flew home the same day, woozy from the side effects.

“It was honestly horrible,” said Reyes, who is a member of the Fridays for the Future movement. Like Nyandire, she had been told to get vaccinated through her country’s national program. But Reyes is 19 years old, and in her small town of Coronango, in central Mexico, there was a limited supply of vaccines available only to older people.

Asked for a response to the confusion, United Nations officials noted that Britain is running the vaccination program and referred questions to the British organizers.

“I am confident everyone who has asked to get vaccinated will get vaccinated,” Sharma insisted.

Whether or not delegates are vaccinated, those coming from countries that Britain has placed on its “red list” because of high infection rates must quarantine upon arrival. Those who are vaccinated like Reyes must quarantine for five days, while unvaccinated travelers must isolate for 10 days.

Under pressure from civil society groups, which argued the cost of quarantine was prohibitive and that the conference should be delayed, Britain has said it will pay for quarantine hotels.

Meanwhile, everyone attending the conference will be expected to show a negative result each day from a self-administered rapid coronavirus test.

For 20,000 delegates over 14 days, that’s potentially 280,000 rapid test kits to be distributed to hotels and private apartments housing delegates. Anyone who tests positive will be asked to immediately isolate and take a PCR test, which must be processed by a laboratory. Results can take more than 24 hours, if laboratory capacity is available.

But delegates need to reach Scotland first.

Tina Stege, the chief negotiator of the Republic of the Marshall Islands, a country whose very existence is threatened by rising sea levels, is trying to navigate air travel in the COVID era.

Flights out of the Marshall Islands are more infrequent because of the pandemic, and getting to Scotland requires navigating the quarantine regulations of various countries in transit. Once delegates return, they also will be subject to the Marshall Islands’ strict two week quarantine for international travelers.

All Stege said she knows for sure is that her country’s delegation will be smaller than years past.

“It’s really crazy to tell you that at this time, with just 30 days, we are still trying to work it out and figure out exactly how we’re going to make it work,” Stege said. “We’ve gone through plan A, B and C.”

There’s another brewing uncertainty: While Glasgow’s public transportation, pubs and hotels require patrons to wear masks, there are exceptions, like when eating, drinking and dancing. Unlike during the Tokyo Olympics, where athletes stayed inside the protective bubble of the Olympic Village, those attending COP26 will be scattered throughout Glasgow.

Inevitably, the question has come up: Is it necessary to have tens of thousands of people gather in person in order to slow down climate change?

Earlier this year, the government of Sweden commissioned a study that looking at whether technology might permit future U.N. climate summits to be held online.

“This is not going to go away with COVID,” said Richard J.T. Klein, a senior research fellow at the Stockholm Environment Institute who led the study. “Even if we all can meet again in person, I think a question that we should ask ourselves is, ‘Do we want to meet again with 30,000 people in one place?’”