A man receives his third dose of COVID19 vaccine at Sheba Medical Center on July 14, 2021 in Ramat Gan, Israel.

Amir Levy | Getty Images

Israeli health officials plan to offer booster shots of the Pfizer-BioNTech Covid-19 vaccine to people over age 60 as the shot’s effectiveness appears to wane as the delta variant spreads across the world, NBC News confirmed Thursday.

The heads of health maintenance organizations that have been administering the Pfizer vaccine will begin administering third shots Sunday, according to NBC News. The booster shots are available for patients above 60 who have already received their second shot at least five months earlier.

The country’s Health Ministry reported last week that the two-dose vaccine is now just 39% effective in Israel where the highly transmissible delta variant is the dominant strain. The shot still works very well in preventing people from getting seriously sick, Israeli officials said, demonstrating 88% effectiveness against hospitalization and 91% effectiveness against severe illness.

The data out of Israel, which began vaccinating its population ahead of many other countries, is bolstering arguments from drugmakers that people will eventually need to get booster shots to protect against emerging variants.

Pfizer CEO Albert Bourla on Wednesday doubled down on his comments that people will need a third dose of the vaccine to maintain its high level of protection against the virus. The U.S. drugmaker published new data Wednesday from a company-funded study that showed the vaccine’s efficacy dropped to about 84% after four to six months.

“We have seen also data from Israel that there is a waning of immunity and that starts impacting what used to be what was 100% against hospitalization. Now, after the six-month period, is becoming low 90s and mid-to-high 80s,” Bourla said on CNBC’s “The Exchange.”

The Centers for Disease Control and Prevention, the Food and Drug Administration and the World Health Organization have said they don’t recommend Covid booster shots at this time, citing a lack of data. U.S. and world health officials have said they are looking at the Israeli research, which was not peer-reviewed and was scant on details.

“We have to be mindful that, with time, the effectiveness of these vaccines may wane,” Dr. Isaac Bogoch, an infectious disease professor at the University of Toronto, said in a recent interview.

He stressed that the shots still appear to be highly effective in preventing severe infection, helping hospital systems not get too overwhelmed heading into the colder months. That being said, “we’re still in the Covid era and anything can happen,” he said.

“We have to be prepared and we have to be nimble that people may need a booster at some point,” he added. “This close surveillance that’s happening in countries like Israel, the U.K. and other parts of the world is going to be very helpful in driving policy if and when we do need boosters.

Israel’s plans to boost its population come two days after the CDC reversed course on its prior guidance and recommended fully vaccinated Americans who live in areas with high Covid infection rates begin to wear face masks indoors again. The guidelines cover about two-thirds of the U.S. population, according to a CNBC analysis.

While the delta variant is hitting unvaccinated people the hardest, some vaccinated people could be carrying higher levels of the virus than previously understood and are potentially transmitting it to others, CDC Director Dr. Rochelle Walensky said Tuesday.

Walensky added new data shows the variant behaves “uniquely differently from past strains of the virus,” indicating that some vaccinated people infected with the delta variant “may be contagious and spread the virus to others.”

“This pandemic continues to pose a serious threat to the health of all Americans,” Walensky told reporters on a call. “Today, we have new science related to the delta variant that requires us to update the guidance regarding what you can do when you are fully vaccinated.”

– CNBC’s Kevin Stankiewicz contributed to this report.