Nobody is eager to be stuck by a needle twice, so naturally many would view a COVID-19 vaccine that provides disease protection after a single injection as a good thing.

Two new studies released Thursday suggest that might be possible.

Both studies involved rhesus macaque monkeys. In one study, researchers from Beth Israel Deaconess Medical Center and pharmaceutical company Janssen injected the animals either with a vaccine candidate or an inert placebo. Six weeks after they were injected, the researchers exposed the animals to the coronavirus, both by putting it in their noses and down their throats.

All 20 of the animals receiving the inert, placebo injection showed signs of infection in both their lungs and noses after being exposed to the virus. But one of the vaccine candidates seemed quite effective in preventing infection. In the six animals vaccinated with this particular candidate, none had signs of infection in their lungs, and only one had a sign of infection in the nose.

Candidate Janssen's parent company, Johnson & Johnson, has decided to start testing in humans. Initial trials started this week.

The vaccine is what's known as a viral vector vaccine. It uses a harmless virus to transfer genetic material from the coronavirus into the person being vaccinated. It's an approach the company has been using for years.

"We have now vaccinated 80,000 people with the vector in different diseases," said Paul Stoffels, chief scientific officer of Johnson & Johnson. And it's known to be safe.

In the other study, researchers at the National Institute of Allergy and Infectious Diseases tested another viral vector vaccine, this one developed by the University of Oxford. Some animals receive a single vaccination; some received two. As in the other study, researchers challenged the monkeys with the virus and waited to see what happened.

None of the vaccinated monkeys developed disease, but all of them still showed signs of active infection in their upper airways.

As of July 1, about 8,000 human volunteers had participated in a study of the Oxford vaccine. If it is shown to prevent disease in humans, that of course would be a good thing. But if it doesn't prevent infection of the upper airway, it means a vaccinated person could still spread the disease.

A third study published earlier this week, also involving macaques, suggested a vaccine being developed by the National Institutes of Health and the biotech company Moderna shows evidence that it too could prevent COVID-19.

"I think it's encouraging," said Carlos del Rio, professor of medicine and epidemiology at Emory University School of Medicine, of the studies in macaques. "But at the end of the day, it's only animal models."

He said only when the human trials have results will we know for sure if any of the vaccines being tested will actually work.

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Tags: COVID-19 vaccine  animal research  coronavirus  Johnson & Johnson