A single shot of the Pfizer/BioNTech COVID-19 vaccine may be less effective than pharmaceutical companies had touted, Israel's coronavirus czar has warned, as he blamed an increase in virus cases partly on a new European variant, The Telegraph reported Wednesday.

Israel reported a record 10,000 new COVID cases Tuesday. In remarks reported by the Israeli Army Radio, Nachman Ash, who heads the country's COVID-19 vaccine program, said one dose appeared "less effective than we had thought," and lower than the drugmaker had suggested.

Pfizer/BioNTech has claimed that clinical tests show the vaccine is almost 52% effective around 12 days after a patient is given the first jab, BBC News reported. But the level of protection increases to 95% roughly two weeks after the second shot.

The government has already administered the first of two shots to more than a quarter of Israel's population and announced Tuesday it would extend eligibility to people who are 40 years old and above.

Doubts over the safety and effectiveness of the vaccine were raised following reports that thousands of Israelis were still falling sick after getting the vaccine, though the country's director for public health, Sharon Alroy-Preis, said that in many cases this was because the patients had not produced enough antibodies after being immunized prior to being exposed to the virus.

Scientists have repeatedly warned that a single shot of the vaccine is not sufficient to provide full protection and that social distancing protocols must still need to be enforced to get the pandemic under control.

Ash, who is Israel's equivalent of the United States' Dr. Anthony Fauci, reportedly warned during a meeting with top government officials that a new COVID-19 variant originating in Europe was hampering efforts to curb the virus, as it was responsible for almost 40% of the new infections.

Over 2 million Israelis have already received their first Pfizer shot, while 400,000 have been administered with a second.

Israeli Prime Minister Benjamin Netanyahu said during the Tuesday cabinet meeting that the country is in "a tight race between the immunization campaign and the high rates of infection in the world because of the mutation."