An hour before Gloucester County opened its vaccination clinic offering third COVID-19 shots Tuesday, two masked men standing at a distance from each other in an empty parking lot marveled at people who don’t want to get vaccinated.

“We don’t know what’s in a shot?” George Perry said, “We don’t know what’s in a hot dog either.”

The Mullica Hill 72-year-old is so sold on vaccines, he turned up long before the clinic opened to get a third Pfizer shot, just authorized for people with severely suppressed immune systems. He arrived armed with the box that holds his leukemia medication, in case anybody questioned his eligibility.

Last week federal authorities said anyone with a health condition that keeps their body from mounting a full immune defense after the regular two-dose vaccine course could get a third dose of Pfizer or Moderna, whichever one they originally had.

Additional shots are not yet recommended for people who received the Johnson & Johnson vaccine.

Because of conditions such as cancer, organ transplant, and HIV, immunocompromised people, roughly 3% of the population, may have had a weak response to their original vaccine doses and could need the extra shot to boost their protection against COVID-19, surging again due to the delta variant.

Third doses may soon be available to everyone already fully vaccinated. The Biden administration is reportedly going to announce all vaccinated Americans will be eligible for a booster shot eight months after their last dose. The doses could be administered as soon as mid-September.

» READ MORE: COVID-19 booster shots are on the way for more of us. Here’s what you need to know.

The policy shift is based on concerns over both delta’s transmissibility and also the vaccines’ waning effectiveness, said Julie Swann, a professor of biomedical engineering at North Carolina State University. Breakthrough cases -- those in people who are fully vaccinated -- suggest the vaccines are losing power over time, Swann said.

“I think that there is growing evidence that there are more breakthrough cases than we know about,” she said. “I don’t even think it’s so rare.”

Breakthrough cases tend to be much milder than those in the unvaccinated -- and are far less likely to lead to hospitalization. But the decline in vaccine potency may be most pronounced in people over age 65, she said.

The World Health Organization recommended this month a moratorium on booster shots to provide more doses to countries that do not have the plentiful vaccine supply the United States enjoys. About 24% of the world’s population is fully vaccinated, according to Johns Hopkins University’s Coronavirus Resource Center.

Doses already distributed to American cities and towns, Swann said, can’t easily be relocated to another country, and should be used for third doses or booster shots.

For now, the severely immunocompromised are the greatest concern.

Up to half of these patients, particularly those with organ transplants and blood cancers, failed to make any antibodies to the coronavirus after vaccination, according to recent studies. Antibodies are the first line of defense against infection, although a study led by University of Pittsburgh researchers detected T cells, a later line of defense, in patients who failed to develop antibodies.

A third vaccine dose, while no guarantee of protection, helps at least some people. Still, there’s no easy way to tell whether the third shot is effective. The government did not recommend measuring antibody levels after the extra dose because the threshold for protection has not been established, and rapid antibody tests are not very reliable.

”This remains a major deficit in our knowledge,” said Ghady Haidar, an infectious-disease and transplant specialist at the University of Pittsburgh Medical Center. “It’s awful that we haven’t figured out all the science. But [offering a third shot] does seem like the right thing to do. In the face of a raging pandemic fueled by the delta variant, we should not let perfect be the enemy of good.”

Hospitals around the region began reaching out to immunocompromised patients to schedule third doses last week. Some county health departments, like Gloucester, targeted such people in promotional materials for clinics. Others are providing third doses, but letting health-care providers take the lead in administering them.

“Most of the people who meet the CDC/FDA definition for this program are associated with healthcare institutions that know who they are, know how to contact them, and have the capacity to administer vaccines to them,” said James Garrow, spokesperson for the Philadelphia Department of Public Health.

The city administered 109 third doses over the weekend, he said.

» READ MORE: The FDA approves an extra COVID-19 vaccine for those with weak immune systems

When the Gloucester County clinic opened its doors at noon Tuesday, six of the seven people already in line were there for a third shot. One 30-year-old was eligible due to medication she takes to manage Crohn’s disease. Another woman who works as a home health aide has gastritis, which she hoped would get her another dose.

“I got to protect my clients,” said Florence Russell, 59, of Clayton. “I go to their homes.”

No one there had to prove their eligibility, officials said.

“We’re not prescreening at all,” said Chad Bruner, the county administrator. “We take them on their face value.”

The only documentation required, he said, was proof of vaccination so that a record of the third shot could be added.

The decision to widen eligibility for booster shots to all vaccinated Americans comes with unanswered questions -- including who should be prioritized to receive them and how effective booster shots may be especially since it isn’t clear whether the vaccines are losing effectiveness generally or if delta can evade the vaccine. Or perhaps both factors are at play.

Kirsten E. Lyke, professor at the Center for Vaccine Development and Global Health at the University of Maryland School of Medicine, is near the end of a National Institutes of Health-funded study of how to make booster shots most effective.

All the 950 participants in her study, which is likely to be reviewed by federal decision-makers, received doses at different times over the last eight or nine months. They also had their antibodies tested prior to receiving an additional dose, offering a look at exactly how the vaccine’s protection wanes over time.

The study also is looking at mixing vaccine types -- for instance, giving a Pfizer shot to someone who had the single-dose Johnson & Johnson vaccine. Mixing doses appears safe, said Lyke, who may release findings for peer review by the end of the month.

“There’s some early evidence that mixing the platforms might even have an enhanced immune response than having the same booster,” Lyke said.

But George Perry, eager as he was to get his third shot, says he’s not going to push the limits of science -- and his immunity.

“I’m acting as if I hadn’t had a shot at all,” he said, vowing to keep up his health precautions. “It’s not a big stretch for me to stay at home.”

Staff writer Marie McCullough contributed to this article.