Israel's health ministry has released data indicating the COVID-19 vaccine from Pfizer may have less efficacy than first thought in preventing infection against the so-called Delta variant of the novel coronavirus.

Data collected during the past month in Israel suggests that the Pfizer vaccine is 64 percent effective at preventing infection from the Delta variant. This compares to 94 percent efficacy against earlier strains of the virus.

Israel's health authorities said the Delta variant is now responsible for more than 90 percent of recent COVID-19 cases. The country reported 501 new cases on Monday, a 50 percent increase on the previous day, and 42 percent of those new cases occurred among vaccinated people.

The results were bleaker than those from another real-world study published in late May by Public Health England that showed the Pfizer jab was 88 percent effective at preventing infection from the Delta variant, which was down from 94 percent.

The Israel study found that Pfizer efficacy at preventing hospitalization had dipped to 93 percent between early June and early July, from 98.2 percent the previous month.

A senior official from the Israeli health ministry said the data was concerning, according to news outlet Ynet. The official said the vaccine is probably less effective for infections and mild illness and that the authorities may consider lengthening the testing period for returnees from abroad, and will take the data into account when coming to a determination on a potential booster vaccination program involving third shots.

Cases of the Delta variant have surged in many nations in recent weeks, including the United Kingdom, where the strain is now responsible for 95 percent of all new cases. There were 50,824 confirmed new cases of the variant during the week ending Friday, bringing the total number of cases of the Delta strain detected in the UK to 161,981, according to Public Health England.

Studies suggest that the Delta variant is 60 percent more transmissible than the UK's previous most dominant strain, Alpha, which itself was more transmissible than the original strain of the novel coronavirus that triggered the pandemic.

The UK government said on Monday that daily cases of COVID-19 could reach 50,000 by mid-July.

Despite the rising number of infections, the UK's prime minister, Boris Johnson, said the government is set to enter step four of its road map for eased restrictions from July 19. This would allow all venues to reopen with no capacity caps, and put an end to social distancing, indoor mask mandates, and limits on the size of gatherings.

Johnson said on Monday that, while "this pandemic is far from over", the majority of those admitted to hospital with COVID-19 in the UK are unvaccinated. He said the UK's vaccination campaign has "greatly reduced mortality". A final determination on the transition to step 4 will be made on July 12.