A fourth dose of the Pfizer and Moderna COVID-19 vaccines has "only marginal" benefits for healthy young people, a new study has found.
The study, which was published in the New England Journal of Medicine, looked at healthy, young health care workers in Israel. It compared workers who had just three shots to those who were given a fourth dose around four months after their first booster.
The research showed that while it restored antibody levels to the previous peak, it didn't really push them any higher, suggesting the maximum immune response is seen after three doses.
The study was carried out when Israel was in the middle of an Omicron outbreak. It found getting an extra Pfizer shot reduced the rate of infection by just 30 percent compared to two vaccines and a booster.
The results were even worse for Moderna with a fourth dose offering just an 11 percent reduced risk of infection.
But the added booster did seem to offer extra protection against symptomatic infection.
According to the study, the healthcare workers who got a second Pfifer booster shot were 43 percent less likely to be symptomatic than those who hadn't.
For Moderna, they were 31 percent less likely to be symptomatic.
The research showed whether workers had three or four doses, most who become infected had minor symptoms.
"A fourth vaccination of healthy young health care workers may have only marginal benefits," the study's authors concluded.
Two vaccines and a booster shot are currently available for most people in New Zealand.
Some severely immunocompromised people are eligible for a third primary dose. The dose, which is different from a booster, is optional but recommended for people aged 12 and over who are severely immunocompromised.
