A third wave of English cases has been driven by the Delta strain among both unvaccinated people—especially those aged 12 to 24—as well as some people who had received COVID shots, according to authors of a study of samples from about 98,000 people in England. The effectiveness of vaccines at stopping infection during the study period fell to 49%, the researchers estimated, down from 64% in a month earlier. Vaccines’ protection against development of COVID symptoms was 59%, down from 83%.

“Development of vaccines against Delta may be warranted,” in the light of evidence that the strain’s spike protein has mutated to a point where antibodies raised by current shots are becoming less effective, the researchers said.

The U.S., U.K. and Israel are among the most fully vaccinated countries in the world, yet all have seen surges in COVID cases and hospitalizations linked to the Delta strain. Health officials in the U.S. have said they’re pleading with the hesitant to get immunized to try to control the spread of the virus, which has potential to lead to further, more dangerous mutations.

Despite the losses, current vaccines continue to give a relatively high degree of protection, the researchers from institutions across the U.K. said. Fully immunized people were three times less likely to catch Delta than non-immunized peers, and less likely to suffer bouts of symptomatic COVID-19 or to pass the virus on to others if they did get infected, according to the study.

The study, called React-1, looked at results of COVID testing from June 24 to July 12. The period roughly corresponds to a surge in infections across the U.K. as the Delta variant came to fully displace the alpha strain that was first detected in the country’s south and caused last winter’s horrors.

Breakthrough infections among fully vaccinated people are becoming an increasingly important issue in countries with high vaccination rates. Such concerns still affect just a small portion of the world as only 13% of people are fully vaccinated globally, most of them in the developed world, the authors noted.

Infections during the period of the study centered more than ever on the country’s youth, with about half of the positive swabs coming from people aged 5 to 24. That age group only accounts for a quarter of England’s population, according to the report, which hasn’t yet been peer-reviewed.

The age breakdown suggests that interventions targeting younger people could have a “disproportionate” impact on slowing waves of COVID, the authors wrote. By vaccinating people between the ages of 12 and 17, for example, health officials could “substantially reduce transmission potential in the autumn when levels of social mixing increase,” the authors said.

“Our vaccination rollout is building a wall of defense that means we can carefully ease restrictions and get back to the things we love, but we need to be cautious as we learn to live with this virus,” Health and Social Care Secretary Sajid Javid said in a statement regarding the report.