(CBS) — Does size matter when it comes to being vaccinated against COVID? Researchers at Oxford University wanted to find out how well the COVID-19 vaccine protects people of different weights from hospitalization and death.

Scientists studied the medical records of more than 9 million people across the U.K. and looked at body mass index, or BMI, a measure of body fat based on height and weight. Dr. Carmen Piernas lead the study and says not everyone is the same.

“The first key outcome is that vaccines are highly effective across all body weights and that is great news,” Piernas said. “In the vaccinated people we see an increase risk of severe COVID in the high weight and low body weight.”

But why those groups are more affected is still a mystery.

“There is something going on with underweight and overweight people in terms of their immunity in terms of their response to the vaccines that we still don’t understand very well that makes them at higher risk of severe COVID-19,” Piernas said.

Researchers also discovered underweight people were less likely to get the shot, but don’t know why. Dr. Piernas now hopes their findings will help governments target the most vulnerable. And for everyday folks, she offers this recommendation.

“The take home messages are basically, take the vaccine if you can, especially if you’re underweight and try to lose weight if you’re over weight,” Piernas said.

Researchers say the next step is to look at the effectiveness of booster shots in people who are obese or underweight.