The Trump administration on Tuesday issued a waiver for lifesaving medicines and medical services, offering a reprieve for a worldwide H.I.V. treatment program that was halted last week.

The waiver, announced by Secretary of State Marco Rubio, seemed to allow for the distribution of H.I.V. medications, but whether the waiver extended to preventive drugs or other services offered by the program, the President’s Emergency Plan for AIDS Relief, was not immediately clear.

Still, PEPFAR’s future remains in jeopardy, with potential consequences for more than 20 million people — including 500,000 children — who could lose access to lifesaving medications. Without treatment, millions of people with H.I.V. in low-income countries would be at risk of full-blown AIDS and of premature death.

“We can very rapidly return to where the pandemic is exploding, like it was back in the 1980s,” said Dr. Steve Deeks, an H.I.V. expert at the University of California, San Francisco.

“This really cannot happen,” he said.

On Monday, the Trump administration ordered health organizations in other countries to immediately stop distributing H.I.V. medications purchased with U.S. aid. The directive stemmed from a freeze — which may become permanent — in the activities of PEPFAR, a $7.5 billion program overseen by the State Department.

Since it started in 2003, PEPFAR is estimated to have saved more than 25 million lives; more than 5.5 million children have been born free of H.I.V. who otherwise would have been infected.

In South Africa alone, PEPFAR’s shutdown would add more than a half million new H.I.V. infections and more than 600,000 related deaths over the next decade, according to one estimate.

The organization employs 270,000 doctors, nurses, pharmacists and other health workers. They had been told not to report to work or to serve patients.

PEPFAR’s end would “create instability and potentially collapse several countries’s AIDS programs that will be difficult to repair, if and when PEPFAR funding becomes available again,” said Dr. Salim Abdool Karim, an infectious disease epidemiologist at the University of KwaZulu-Natal in Durban, South Africa.

Dr. Abdool Karim said countries should stop relying on PEPFAR and support their own citizens, a goal that the program’s staff and partners had been working toward. But ideally that shift would happen gradually, over years during which PEPFAR would train local health workers and prepare them for the transition, he said.

“This is not a bad opportunity for countries to take greater responsibility,” he said. “But I think they can’t do it if it’s done in this kind of haphazard and unplanned way.”

Here’s what he and others expect from PEPFAR’s unexpected pause.

Sudden stops to H.I.V. treatment can quickly turn dangerous.

Every day, more than 220,000 people pick up H.I.V. medications at clinics funded by PEPFAR; the number included more than 7,400 children under 15, according to data published on Tuesday by AMFAR, The Foundation for AIDS Research.

The drugs work by suppressing H.I.V. in the body. When patients go off the drugs, the virus grabs the opportunity to rebound — and quickly. Within a week, H.I.V. levels will skyrocket from undetectable levels to more than 100,000 copies per milliliter of blood.

“That may be a time where you are very much at risk of passing the virus on to others,” Dr. Sallie Permar, a pediatrician and H.I.V. expert at Weill Cornell Medicine, said.

Then, the virus will start attacking a certain type of immune cell, crippling the body’s ability to fend off other infections, including tuberculosis, which frequently accompanies H.I.V. infection.

Spiking H.I.V. levels at first may cause flulike symptoms, including sore throat, swollen glands and fatigue. The immune system will likely marshal enough force to suppress the virus temporarily, but H.I.V. is adept at hiding until it finds the right opportunity to re-emerge.

When that occasion arises, “they can develop AIDS and progress,” Dr. Deeks said.

Children may be among the hardest hit.

PEPFAR is best known for financing H.I.V. treatment programs, but its funds also go to drugs for prevention, outreach and testing, and to support for orphans and women experiencing gender-based violence.

The loss of resources for each of these efforts will derail the fight against AIDS, said Dr. Glenda Gray, a pediatric H.I.V. expert at Wits University in South Africa.

“If H.I.V. testing falls by the wayside, it’s unlikely that we will be able to even diagnose people who need to go into treatment,” she said.

If a pregnant or breastfeeding woman has H.IV. but is not tested and not treated, she may pass the virus to her child. The higher her viral load, the more likely this is to occur.

Children with H.I.V. are less likely to be diagnosed than adults, and may not be treated till the virus makes them visibly very sick. This progression can be much more rapid in children than in adults, Dr. Gray said, “and obviously, children who are untreated are likely to die.”

Inconsistent treatment drives drug resistance.

As people lose access to medications, they may try to spread out their supplies by alternating days or to share their pills with others. If the virus replicates in people with only partial protection, it can learn to evade those defenses and become resistant to the medications.

People living with the virus may then pass the resistant virus on to others.

“That becomes a big problem, because now, suddenly, our cheap first-line drugs might not work when we have to restart them on treatment,” Dr. Abdool Karim said.

A virus that is resistant to treatments will also be better at evading vaccine candidates being tested.

“Not only are we looking at more drug resistance, but we’re looking at losing whatever ability we had to make an effective vaccine,” Dr. Permar said.

PEPFAR’s end may affect Americans, too.

More than one million Americans are living with the virus, and more than 30,000 become infected each year. If H.I.V. becomes resistant to available medications, it isn’t likely to remain in low-income countries. Americans, too, will be at risk.

They may also face indirect harms from ending PEPFAR. Creating huge populations of immunocompromised people may mean that other pathogens have an opportunity to spread. For example, dangerous Covid variants, including Omicron, are thought to have evolved in immunocompromised people with H.I.V.

At the same time, people worldwide have benefited from trials conducted under PEPFAR’s auspices, showing the importance of treating H.I.V. early, demonstrating that pregnant women can safely breastfeed as long as they are treated and that H.I.V. infections can be prevented with long-acting drugs.

“America has gotten an amazing amount of love around the world because of what it’s done,” Dr. Deeks said.

“From a humanitarian perspective, I can’t imagine anyone really wants to go along this pathway,” he added. “This doesn’t make any sense on any level.”