Protection against the coronavirus is waning among those who have received both shots of the AstraZeneca and Pfizer vaccines, a new U.K. study has found.

An analysis from the U.K.’s ZOE Covid app study of over 400,000 people who had received both shots of the Pfizer–BioNTech vaccine, showed that it was 88% effective in protecting against the coronavirus a month after receiving both shots. However, its effectiveness fell to 74% five or six months after receiving both doses of the Pfizer vaccine.

The data was collected after May 26, when the delta variant became the dominant strain, said Tim Spector, who is running the ongoing ZOE Covid app study.

Spector, a professor of genetic epidemiology at King’s College London, presented this latest data during a webinar on Tuesday and said that the findings showed a “reduced benefit” of protection from both of these vaccines as the months progress.

Nearly 42 million people in the U.K., 77% of the population aged over 16, have received two doses of a Covid vaccine, according to government data last updated on Tuesday. The daily data showed 30,838 Covid-19 infections had been recorded on Tuesday, while 174 people had died within 28 days of testing positive for the virus.

Alexander Hammers, a professor of imaging and neuroscience at King’s College London, said on the webinar it was already known from other coronaviruses that immunity against the disease didn’t tend to be lifelong.

And even though the data did show a “waning” in the effectiveness of the vaccines over time, Hammers said that people were “still probably at least 50% protected.”

“Remember when the vaccines were first developed it was hoped that they were to have 60%-70% efficacy and everybody was pleasantly surprised that they came in well over 80%, sometimes well over 90,” he added.

Nevertheless, Spector said he was still “a bit worried” by coronavirus data coming out of Israel, which had one of the fastest vaccination programs in the world and was ahead of the U.K.

He pointed out that Israel was starting to see increased hospitalizations and deaths from the coronavirus, despite a large proportion of its population being vaccinated.

Data published by Israel in July, showed that the Pfizer vaccine was just 16% effective against symptomatic infection for those individuals who received two doses in January.

Comparing results is tricky, however, given differences in the nature of the vaccination programs in different countries, as well as differences in study dates, age groups and Covid testing regimes.