Updated: Aug 28, 2021
Israel was one of the first countries in the world to achieve significant vaccination rates amongst their population (with approximately 60% of their population being fully vaccinated as of end of July 2021).
Many of their most vulnerable populations (older than 60, immune compromised) were vaccinated in January to February 2021. However, due to the rapid spread of the delta variant which first emerged in India and was responsible for their second wave beginning February 2021, Israel is currently experiencing a significant increase in daily cases.
Based on a recent study (Mizrahi et. al.) published on July 31, 2021, Israeli scientists have found that those vaccinated early (in January and February, 2021) are 53% more likely (consistent trend across age groups) to suffer from a breakthrough infection versus those vaccinated later (March and April, 2021) during the month of July, 2021.
Thus, the efficacy of two shots of BioNTech/Pfizer mRNA BNT162b2 vaccine appears to wane over a 5 - 6 months period.
It should be noted that according to Israeli government data (as of July 18, 2021), the Pfizer vaccine is still up to 88% effective in preventing hospitalizations, and up to 91.4% effective in preventing severe COVID-19 after approximately 6 months from the second dose.
However, due to the increasing in breakthrough cases, Israeli data shows that the Pfizer vaccine is only 39% effective at preventing infection and 40.5% effective at preventing symptomatic COVID-19 after approximately 6 months from the second dose.
Based on the above findings, vulnerable groups who were vaccinated early in January and February 2021 should consider getting a booster dose at around the 6 months mark in order to increase protection. Early data from those vulnerable populations in Israel who have already received a third booster dose is promising, with those receiving a third dose up to 2.5 times more protected than those who received two doses.