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Pfizer Vaccine's Second Dose

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Two new discoveries could rapidly speed up vaccination efforts against COVID-19 worldwide.

Data from Israel suggests that delaying the Pfizer vaccine's second dose has only a small impact on efficacy. This could effectively double the number of people protected while we wait for more vaccines to be made, since second doses could be given to people still waiting for their first shot.

The other finding shows that the Pfizer vaccine is stable even when kept at normal freezer temperatures — potentially making it much easier to distribute the vaccine.

Delaying Pfizer Vaccine's Second Dose

In November, Pfizer reported the results of a phase 3 trial in which volunteers received two doses of its COVID-19 vaccine, delivered 21 days apart.

According to that trial, the vaccine was 95% effective at preventing symptomatic infection seven days after the second dose. The efficacy before the second dose was 52.4%.

However, because the immune system takes a while to fully respond to a vaccine, almost all of those infections in the vaccine group happened in the first two weeks after the first shot. After that, the vaccine appeared to give very strong protection, "as high as 80 or 90%," according to experts who analyzed the data in December — even without a second dose.

Now, a new peer-reviewed study suggests that the Pfizer vaccine's second dose really might not make a huge difference, at least in the short-term.

The study involved 9,000 members of the medical staff at Israel's Sheba Medical Center. About 7,000 of the participants received a single dose of the vaccine. The rest remained unvaccinated.

Based on the number of infections, the researchers determined that the single dose was only 47% effective at preventing a symptomatic COVID-19 the first 14 days — similar to the phase 3 study results.

However, between 15 to 28 days after, it was 85% effective — almost as high as the two-dose regimen in Pfizer's trial.

"The study shows early effectiveness, even before the second dose was administered."

Eyal Leshem

The study didn't determine whether protection lasts as long without the Pfizer vaccine's second dose, so long-term immunity may depend on getting the booster. It also didn't include as many people over the age of 65 as Pfizer's trial, so it's unclear whether the single-dose regimen is equally effective in all demographics.

However, this study does support arguments that we should be distributing available vaccines to as many people as possible before administering second doses, a strategy the U.K. has been pursuing. This would effectively double the number of people who could be protected with the same supply of vaccines.

"This is the first study assessing effectiveness of a single vaccine dose in real-life conditions and shows early effectiveness, even before the second dose was administered," study co-author Eyal Leshem told the Wall Street Journal.

Not-So-Cold Storage

The second welcome discovery surrounding Pfizer's COVID-19 vaccine comes directly from Pfizer itself.

On February 19, the company announced that the vaccine can be stored at temperatures between -13 and 5 degrees Fahrenheit — not the ultra-cold -112 to ‑76 degrees previously thought.

That means the COVID-19 vaccines needn't be in specially designed freezers — pharmacies can hold them in the freezers they typically already have on site.

"This would offer pharmacies and vaccination centers greater flexibility."

Albert Bourla

Pfizer has now submitted data backing up this claim to the FDA in the hope of getting its Emergency Use Authorization updated with the new temperature requirements.

"We appreciate our ongoing collaboration with the FDA and CDC as we work to ensure our vaccine can be shipped and stored under increasingly flexible conditions," Pfizer CEO Albert Bourla said in the announcement.

"If approved, this new storage option would offer pharmacies and vaccination centers greater flexibility in how they manage their vaccine supply," he continued.

Combine this with the news that Pfizer vaccine's second dose might not be as immediately necessary as thought, and we could be looking at an opportunity to protect more people in more places from the coronavirus sooner than previously believed possible.

Updated, 02/24/21, 3:15 ET: This story was updated to correct the link to the Lancet study.

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