The Pfizer-BioNTech coronavirus vaccine has dropped to 64 percent effectiveness in preventing infection in Israel as the delta variant continues to spread across the country, the Health Ministry said on Monday.

According to Israeli data published in March, two weeks after the vaccine was given, it had 99 percent efficacy in preventing symptomatic COVID-19 and 91.2 percent in preventing infection. 

The ministry added that the vaccine is 93 percent effective in preventing hospitalizations and severe symptoms.

The ministry refrained from recommending administering a third dose of the vaccine at this stage. It, however, called on all those who came into contact with a confirmed COVID-19 carrier, including vaccinated ones, to get tested for the virus.  

The data was presented on Sunday by members of the Health Ministry's epidemiology team. However, experts have expressed doubt regarding the analysis of infection rates in Israel in recent weeks and the ministry's epidemiology team has also questioned it.

Nevertheless, health sources believe that the inoculation's efficacy against the delta variant is much lower than initially presumed.

The models used by the Health Ministry have been called into question by health professionals. 

"There are enormous methodological challenges in properly assessing the efficacy of the vaccine from local outbreaks," one health expert told Haaretz.

Prof. Ran Balicer, the chief of the professional coronavirus cabinet which advises the Israel's coronavirus cabinet, added that it is very difficult to gauge vaccine efficacy when COVID tests are performed selectively.

"This is fundamentally different from the evaluation that came from the U.K.," another expert stated. 

The Health Ministry's model was built on the assumption that the vaccine is 80 percent effective against the delta variant, as opposed to 90 percent against the alpha variant hailing from Britain.

The main research findings until now, including those by British health authorities published in the medical journal Lancet, show that two weeks after the second dose is administered, the Pfizer-BioNTech coronavirus vaccine is 88 percent effective against the delta variant. That is only slightly less than the 93 percent for the alpha variant.

The delta variant, however, is considered to be one and a half times more contagious than the alpha, and twice as infectious as the original version of the virus.