Is the Pfizer Vaccine Really 100% Effective in Teens?

Pfizer said earlier this week that its vaccine is 100% effective in teenagers over the age of 12 but has previously said it's 95% effective in adults.

NBC10 Boston asked three top Boston doctors on Tuesday to explain that discrepancy and if the data would help to boost the chances of Pfizer's vaccine for kids getting full approval from federal regulators in a weekly series, "COVID Q&A."

What does 100% effectiveness really mean?

Pfizer's study on vaccines in children ages 12 to 15 involved 30 cases of COVID in the entire sample. All of the cases occurred in the placebo group, which is why they reported 100% effectiveness, according to Dr. Shira Doron of Tufts Medical Center .

"It's not likely we'll see 100% effectiveness in the real world, but we know that the vaccine is very effective, particularly in the first four months, which is what this study was reporting," she said. "It does not mean that the vaccine is necessarily more effective in this age group than it is in adults."

The studies were not designed to compare the vaccines in children vs. adults, Boston Medical Center's Dr. David Hamer explained.

"These are based on relatively small numbers of individuals, not a really large sample size," he said. "As we go and sort of post-marketing data collection, that's going to be really important to assess, you know, how the real-life effectiveness of this vaccine for preventing both infections, and of course, hospitalization and death, but those are going to be rare outcomes in this age group in any case."

Based on the study, the vaccine's effectiveness could be anywhere from some much lower number up to 100%, Dr. Daniel Kuritzkes of Brigham and Women's Hospital said.

"These are just results based on a particular sample with a relatively small number of people in them and it would be unrealistic to expect that real world performance is going to be the same as what we've seen in the trial," he said. "That said, it means that this vaccine, as it was in older adults, is highly effective."

Will this boost Pfizer's chances of getting federal approval?

"I think positive data are always very helpful. You know, it's important that they have that data on actual protection, not just data on inducing high levels of antibody, which is more about correlate protection, not really evidence of protection," Kuritzkes said. "And so these data are going to be very helpful in an application to the FDA for approval."