WASHINGTON — A devastating virus was laying waste to nations that lacked medicines available to Americans. The pills were patented and pricey. Poor countries lacked refrigeration to store them, the thinking went, and patients would not be able to follow the complex dosing regimen.

The year was 2002, the virus was H.I.V., and the president, George W. Bush, secretly sent his top health advisers to Africa to investigate what activists were calling “medical apartheid.” In the 20 years since, the United States has led the way in building a global infrastructure for H.I.V. testing and treatment, saving an estimated 21 million lives.

Now, with that history in mind, global health agencies and the Biden administration are working to bring coronavirus tests and expensive antiviral pills to low- and middle-income nations. This week, President Biden will emphasize “global test to treat” at his second international Covid-19 summit, a virtual gathering of world leaders aimed at injecting new energy into the international pandemic response.

Until now, the response has been focused largely on vaccinations, which remain a high priority. But Mr. Biden will also use the summit to call on wealthy nations to donate $2 billion to purchase Covid treatments and $1 billion to purchase oxygen supplies for low- and middle-income countries, according to a senior administration official involved with the planning.

In the United States, where antiviral pills to combat Covid are widely available, Mr. Biden’s “test to treat” initiative lets many patients go to pharmacies, get tested for Covid and receive a free prescription on the spot if they test positive. In low- and middle-income nations, such efforts will most likely be much more limited until generic pills arrive, probably in 2023.

But the global effort faces some of the same obstacles and inequities that existed two decades ago.

Rich nations, including the United States, have gobbled up much of the supply. Global health agencies do not have the money to buy the antivirals or tests, which are crucial because the medication needs to be started early in the course of infection. Drug companies, trying to protect their patents, are limiting the supply of generic alternatives in many middle-income countries, including an entire swath of Latin America.

All of this is playing out against the infectious disease equivalent of a ticking time bomb.

“We all expect a major new surge from Omicron or a new variant in the global south from June to September, and if that happens, we are not going to be ready with test and treat,” said Dr. Bill Rodriguez, who runs the testing arm of the ACT Accelerator, the Geneva-based consortium coordinating the global response. “It feels extremely similar — painfully, ironically, tragically similar — to what happened with H.I.V.”

On Monday, ahead of the summit, the consortium, which is backed by the World Health Organization, is set to convene a discussion of global health experts and declare access to testing and treatment an “equity issue,” officials said.

Mr. Biden may well show up at his own summit empty-handed. The White House has asked Congress for an additional $22.5 billion in emergency coronavirus aid, including $5 billion for the global response, but Senate Republicans are refusing to authorize any funding unless it is offset by cuts to other programs. A $10 billion compromise proposal includes no money for the global response, and it is unclear when or whether that plan will come up for a vote.

“Here we are with another virus that’s creating havoc all over the world,” said Gayle Smith, who ran the State Department’s global Covid response under Mr. Biden and is now chief executive of the One Campaign, an advocacy organization. “Maybe it’s not as lethal as H.I.V., but it’s still pretty bad. And we’ve got therapeutics. How are we going to think about this? Are we going to take the lessons learned and have a plan?”

One of the biggest hurdles is the rapid decline of Covid testing around the world. The W.H.O.-backed consortium recently reported that just 20 percent of the 5.7 billion tests conducted globally have been in low- and middle-income nations. Low-income countries accounted for less than 1 percent of the testing. The reasons are twofold: Countries lack money to buy the tests, and demand has dropped in regions where Covid rates are now low.

“What really worries me is the testing part of this,” said Dr. Bruce Aylward, a top W.H.O. official and the consortium’s coordinator. “If you’re not testing, you can’t sequence, you can’t isolate, you can’t treat. Everything else unravels.”

Paxlovid, the more powerful of the two Covid antiviral pills approved by the Food and Drug Administration, is so plentiful in the United States that pharmacies are struggling to use up their supplies. The Biden administration has committed to purchasing 20 million treatment courses for Americans.

The W.H.O. recently issued a “strong recommendation” that Paxlovid, which is made by Pfizer, be given to patients at high risk of hospitalization and called for its “wide geographic distribution.” The W.H.O. has given a far weaker “conditional recommendation” to the other drug, molnupiravir, which is made by Merck and is not nearly as in demand.

Global health experts say both companies have absorbed the lessons of AIDS — but only to a point.

They have each agreed to allocate several million courses of treatment — a total of seven million courses in all — to UNICEF for distribution in most low- and middle-income countries, which account for more than half the world’s population. But UNICEF will not be able to buy the drugs unless it can raise the money to do so or countries supply the funds. And seven million courses is hardly enough to address the need, experts say.

The cost to UNICEF of Paxlovid — including Pfizer’s insistence that UNICEF keep how much it pays confidential — remains a sticking point, said Dr. Philippe Duneton, who runs the therapeutics arm of the W.H.O. consortium. In announcing its “strong recommendation” for Paxlovid, the W.H.O. took the highly unusual step of publicly scolding Pfizer for a “lack of transparency,” which makes it difficult to know which countries have the drug and what they are paying.

“We need to have better visibility in terms of price,” Dr. Duneton said.

Manufacturers often prefer that the details of their sales agreements be secret so as not to weaken their hand with other potential buyers. Pfizer’s chief executive, Albert Bourla, reported last week that Paxlovid had been a “key growth driver” for the company, which is using a “tiered pricing approach” in which low- and lower-middle-income countries will get Paxlovid at a not-for-profit price.

In response to an inquiry from The New York Times, Pfizer issued a statement saying that it was “deeply disappointed by the sentiment expressed by our partners,” adding, “We have in good faith heard and responded to many of their concerns.”

Both Pfizer and Merck have also taken steps to make inexpensive generic versions of their pills available, signing licensing agreements with the Medicines Patent Pool, which was created during the global AIDS crisis to bring drugs to low- and middle-income nations at low cost. It took years, and bitter fights between activists and companies, to reach such agreements for H.I.V. drugs.

But the agreements for Covid antivirals do not apply to many middle-income nations, including much of Latin America and parts of northern Africa and Asia. The result, experts say, is that both poor and rich nations will have access, but countries in the middle will have to negotiate with the companies — or force the drugmakers to turn over their intellectual property.

So far, 36 companies from 12 countries have signed up to make generic Paxlovid. Companies in India are already making generic versions of both Paxlovid and molnupiravir. The expectation is that both drugs will ultimately be available in about 100 low- and middle-income countries, covering about half the world’s population. The companies will not receive royalties from the sales while the W.H.O.’s declaration of the pandemic as a global health emergency remains in effect.

“Given the severity of the pandemic and given the fact that vaccines had a very uneven penetration rate, we felt that this was a very important contribution the company could make,” said Paul Schaper, executive director for global public policy at Merck.

But those generics will not be available until next year. In the meantime, doctors and activists around the world say vulnerable patients are dying as antiviral pills, monoclonal antibodies and even oxygen remain out of reach. In countries with low vaccination rates, the need is especially urgent.

In Uganda, Dr. Sabrina Kitaka, a pediatrician who also advises the government on Covid-19 vaccination, said many children with underlying conditions — sickle cell disease, diabetes, advanced H.I.V. disease — have had complications from Covid-19. She has lost young patients who would have been eligible for Paxlovid, which is approved in the United States for children 12 and older who weigh at least 40 kilograms, or about 88 pounds.

“Paxlovid will be the game changer,” Dr. Kitaka said. That is especially true “for patients who become critically ill and end up in the I.C.U.,” she added.

In the Dominican Republic, Pfizer is fighting a petition for the government to compel the company to share its patents for Paxlovid with generic makers. Similar petitions have been filed in Colombia, Chile and Peru.

“Both Merck and Pfizer have reserved for themselves all the high-income countries and virtually all of the upper-middle-income countries and even some lower-middle-income countries,” said Brook Baker, a law professor at Northeastern University who has submitted a legal brief in support of the Dominican Republic petition.

In Brazil, regulators have authorized both Paxlovid and molnupiravir. The country and Pfizer are negotiating a purchase agreement so Paxlovid can be offered for free through Brazil’s public health system.

But a quarter of Brazilians have private insurance and may already have access to the drug, said Felipe Carvalho, coordinator of the Doctors Without Borders Access Campaign in Latin America.

“We have a world divided between treatment for the rich and treatment for the poor, still today,” Mr. Carvalho said, adding, “We are still struggling 20 years, 30 years after the H.I.V.-AIDS crisis to convince companies to do the right thing.”

In 2003, after he sent top government health experts, including Dr. Anthony S. Fauci, to Africa, Mr. Bush created PEPFAR, the President’s Emergency Plan for AIDS Relief, which supports clinics, medical personnel and community health workers.

Dr. Deborah L. Birx, who ran PEPFAR for seven years and also served as President Donald J. Trump’s coronavirus response coordinator, said that Covid vaccination campaigns relied on the program’s infrastructure and networks and that a “test to treat” effort could too. Delivering drugs in Africa can be easier than in the rural United States, she said.

“Unlike the United States,” Dr. Birx said, “we have shown that we can deliver drugs to the far end of the road, no matter where people live.”

Leaders of the ACT Accelerator consortium are eager to get started. They have plans to expand rapid testing in 20 “priority countries,” mostly in Africa, and to run pilot “test to treat” programs in 10 nations. But both initiatives are in limbo because of a lack of money and a separate reason that officials find even more troubling: a sense of pandemic fatigue in nations whose leaders have other pressing priorities.

“We have never been more ready to roll out global testing and treatment for any disease than we are for Covid right now,” said Dr. Rodriguez, who runs the consortium’s testing arm. But the halting pace saddens him: “It’s painful. It’s really painful.”