Countries are worried that COVID-19 vaccines might be starting to wear off. Despite some concerning signs, it's not yet clear if that's the case.

• Pfizer has suggested that data coming out of Israel means vaccine immunity could be waning.
• But experts told Insider the data is not conclusive.
• Even if immunity did wane, vaccines would continue to protect against severe disease, experts said.

Health authorities are beginning to explore plans for third doses of coronavirus vaccines - raising the prospect that two doses may not be enough.

Last week Pfizer announced that it would be seeking authorization for the third shot of its vaccine. Pfizer's R&D director Mikael Dolsten said that data coming out of Israel could mean the vaccine has started to fade in people vaccinated in January and February.

But the World Health Organization and US public health officials have said that data doesn't support the need for a booster yet.

Data from Israel suggests vaccine efficacy has dropped

Last week, the Israeli ministry of health said that the Pfizer vaccine effectiveness against infection and mild symptoms dropped to 64% in June from earlier highs of above 90%.

The change occurred at the same time as the Delta variant, which is more resistant to vaccines was spreading in Israel, making it hard to say exactly what is behind the change.

The vaccine still reduced the risk of severe disease and hospitalization by 93%, according to the ministry.

Israeli journalist Nadav Eyal, of the Ynet news outlet, published data suggesting fading immunity in the earliest groups to receive vaccines, as shown in this graph:

Eyal attributed the stats to the health ministry, but officials there have not confirmed them as genuine. The graph shows more people vaccinated in January being reinfected compared to those vaccinated in March, for instance.

According to Meaghan Kall, an epidemiologist working for Public Health England, preliminary data from the UK could also indicate that vaccine immunity is waning.

In the graph below, the blue line is infections in a study of health care workers (HCW), 96% of whom are fully vaccinated.

"This could represent waning immunity (HCW were among first vaccinated in Jan/Feb) or vaccine/immune escapee of Delta," Kall said in the following tweet.

Waning immunity, or something else?

There are several plausible explanations beyond immunity waning, Professor Eyal Leshem, an infectious disease specialist at Israel's Sheba Medical center, told Insider

It is also possible that the Delta variant is breaking through the protection given by the vaccines. Israel prioritized vaccinating its most vulnerable, so those vaccinated in January are also more likely to get tested, fearing the consequences of an infection.

Shane Crotty, a virologist at La Jolla Institute for Immunology in California, was not convinced by the Israel data.

Findings released by Pfizer in April showed that the vaccine was still 91% effective against infection six months after the second dose of the vaccine.

"That's about as good as you could possibly hope for," Crotty said.

As recently reported by Insider's Dr. Catherine Schuster-Bruce, data from other countries put the vaccine efficacy much higher than in Israel: 88% effective in a study published in the UK in May, and 87% effective according to a study from Canada published July 4.

"I put a lot of weight on those, as opposed to undisclosed, small-study data out of Israel," Crotty said.

Protection from severe disease is likely to last longer than against any infection

Speaking to Bloomberg, Pfizer's Dolsten said the data from Israel could mean that the vaccine's front-line antibodies have faded.

"When you have low blood levels of an antibody, viruses that are highly contagious may reinfect and cause mild disease," he said.

But the protection against severe disease is likely to last longer because other immune responses are at play, Dolsten said.

A booster dose could bring that level up again, Dolsten said. Preliminary results released by Pfizer suggest a third dose of the vaccine increased the number of neutralizing antibodies against the virus by five to 10 times.

Leshem, the Israeli infectious-disease expert, doesn't think the data from Israel is that concerning:

"Maybe there's a reduction in antibodies and maybe that is a reduction in protection against infection, but there doesn't seem to be a reduction in the most important outcome: protection against hospitalization," he said.