The United Kingdom's chief scientific advisor said the UK will look "very carefully" at recent COVID-19 vaccine analysis out of Israel, which suggests that the level of protection provided by one dose of the Pfizer treatment is notably lower than first indicated in trials.

Israel's health authorities provided some of the first real-world data regarding COVID-19 vaccine effectiveness on Tuesday, releasing figures that suggest the chances of someone being infected by the virus decrease by 33 percent after receiving one Pfizer shot.

This compares unfavorably with clinical trial data, which found the Pfizer vaccine was 52 percent effective after one dose, and 95 percent effective after two.

The analysis could have implications for the UK, which has opted for a strategy that involves delaying second doses beyond vaccine manufacturer recommendations, in order to provide a larger proportion of the population with the first shot.

"We need to look at this very carefully, we just need to keep measuring the numbers," UK Chief Scientific Advisor Patrick Vallance told Sky News on Wednesday. "When you get into real-world practice, things are seldom as good as clinical trials."

Experts have eagerly awaited early vaccine data from Israel, which has impressed with the speed at which it has inoculated more than one-fifth of its population. Israel's National Coronavirus Project Coordinator Nachman Ash told domestic broadcaster Army Radio that the first dose is "less effective than we thought" and that data on effectiveness was found to be "lower than presented by Pfizer".

"Many people have been infected between the first and second injections of the vaccine," Ash said.

But Vallance said that more information is needed before firm conclusions could be drawn, adding the first dose will not provide much protection whatsoever within the first 10 days, as the immune system is yet to develop a response, and also some people are likely to have contracted the virus prior to inoculation.

"I don't know exactly what Israel are looking at," Vallance said. "If they are looking at the total period from day nought, then that doesn't give an exact comparison (with the trial data)."

Stephen Evans, who is a professor of pharmacoepidemiology at the London School of Hygiene and Tropical Medicine, cautioned against reading too much into the preliminary figures from Israel, which were collated from one of the country's main health service providers and presented without a detailed description of methodology.

"It is vital that advice and policy take into account the latest available data," Evans said. "However, the reports that have come from Israel are insufficient to provide any evidence that the current UK policy in regard to delaying the second dose of vaccines is in any way incorrect."

Evans said there is a need for both more data and detail on study methods in order to understand and interpret the findings.

"It is not sensible to compare efficacy derived from an observational study of this type which is subject to many biases, with the efficacy derived from randomized trials," he said.

Evans also said that information on the severity of cases seen among the vaccinated, including data on hospitalization, is necessary in order to assess the policy of delaying a second dose.

"The efficacy against detected COVID-19 may be less important in the UK context than efficacy against hospitalization and death."