The numbers from Israel (https://www.bloomberg.com/news/articles/2021-07-05/israel-sees-decline-in-pfizer-vaccine-efficacy-rate-ynet-says) indicate the Pfizer vaccine is quickly losing efficacy against the Delta strain with only 64% chance to prevent symptomatic infection - but critically, none of these articles covering that report indicate if that's for full 2-dose vaccination.

Another recent study "showed the Pfizer vaccine was 84% effective against the [Delta] variant after two doses, but only 34% effective after the first dose." (https://www.nbcboston.com/news/local/pfizer-moderna-jj-vaccines-efficacy-as-delta-variant-concerns-rise/2419162/). The effectiveness rating appears to be the chance to prevent symptoms.

A slightly earlier article (https://www.businessinsider.com/delta-coronavirus-variant-strongest-threat-vaccinated-people-2021-6) places Pfizer efficacy at >= 88% chance to prevent symptoms with full vaccination, but only 33% for partial vaccination.

Intuitively, the numbers from Israel seem to be an outlier. The critical question: Does that statistic mix full vaccination and partial vaccination?

If the numbers are mixed, that would explain why the efficacy was found to be roughly halfway between partial and full (since some people already had 2 doses while others have only 1). If the numbers are for full vaccination... then it's time to talk about booster shots.

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    Your first link says "The government plans to study vaccinated individuals who contracted the coronavirus, including factors such as age, pre-existing conditions and inoculation dates, in order to evaluate the efficacy of the vaccine and the rate at which it wears off, the prime minister’s office said." That certainly implies such information has NOT yet been evaluated.
    – Armand
    Commented Jul 6, 2021 at 6:26
  • @Armand Good catch. I'm surprised they didn't take into account inoculation dates, because there's a 2-week delay before protection sets in. If inoculation dates and number of doses aren't counted, the 64% protection rate would count dubious cases such as someone who tested positive the day after getting the first dose, which obviously does not represent a breakthrough infection. Commented Jul 6, 2021 at 15:12
  • gov.il/en/departments/news/06072021-04 has plenty of explanations in English (about this particular "64%"), including the Lancet paper/model they've been continuously using for such announcements. That you and some Forber couldn't find it is not Israel's fault. Also you refuting their finding in a self-answer by saying that some other study, using some other methodology, found something else in different environmental circumstances just shows this is not a real question. Commented Aug 30, 2021 at 10:39
  • I would grant you that one of the pdfs attached at the bottom there (with monthly data) is in Hebrew. FYI: there are now some studies from the US coming up with similar findings as Israel did a couple of months ago. (Somewhat time-shifted, but mostly in terms of publication time.) If you read carefully just the abstract of the latter, you'll probably undestand what circumstances are now similar in the US . Commented Aug 30, 2021 at 10:53
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    Separately the CDC also announced a reduction in v. eff. to 66%... edition.cnn.com/2021/08/24/health/us-coronavirus-tuesday/… albeit their study only sampled some kinds of "frontline" workers (healthcare etc.) Unlike Israel they lack the capability to track the entire population in such longitudinal cohort studies, so US studies are either hospital-based (e.g. Mayo, prev link or on selected cohorts like the CDC one.) Commented Aug 30, 2021 at 11:23

1 Answer 1


The New York Times (https://www.nytimes.com/live/2021/07/06/world/covid-19-vaccine-coronavirus-updates) presents several studies indicating good 2-dose protection against symptomatic infection: Britain and Canada reporting 87-88%, and Scotland reporting 79%.

A recent report from Israel claims only 64% protection, but that report is heavily marred by lack of transparency: According to Forbes (https://www.forbes.com/sites/roberthart/2021/07/06/pfizer-shot-much-less-effective-against-delta-israel-study-shows---heres-what-you-need-to-know-about-variants-and-vaccines/), "Israel’s health ministry has not revealed the data or the methodology it used to obtain its results."

Therefore, the most logical conclusion at this time is that full mRNA vaccination continues to provide superior protection, probably in the 80%-90% range. The 64% number should be rejected as a dubious outlier until more transparency is provided.

Updating answer with new data recently in - an article presenting data from the New England Journal of Medicine. The numbers are 88% protection from 2-dose Pfizer vs. Delta (consistent with the above consensus), 67% protection from 2-dose AstraZeneca vs. Delta.

Source (https://www.reuters.com/business/healthcare-pharmaceuticals/two-doses-pfizer-astrazeneca-shots-effective-against-delta-variant-study-finds-2021-07-21/):

The study, published in the New England Journal of Medicine, confirms headline findings given by Public Health England in May about the efficacy of COVID-19 vaccines made by Pfizer-BioNTech and Oxford-AstraZeneca (AZN.L), based on real-world data.

Wednesday's study found that two doses of Pfizer's shot was 88% effective at preventing symptomatic disease from the Delta variant, compared to 93.7% against the Alpha variant, broadly the same as previously reported.

Two shots of AstraZeneca vaccine were 67% effective against the Delta variant, up from 60% originally reported, and 74.5% effective against the Alpha variant, compared to an original estimate of 66% effectiveness.

  • The link is to a news story, not a scientific report. The story itself admits "Israel’s health ministry has not revealed the data or the methodology it used to obtain its results " Thus, for now at least, it's not scientific.
    – Armand
    Commented Jul 7, 2021 at 5:13
  • @Armand I found a better article from the New York Times, and edited the answer to show how the numbers from Israel are dodgy Commented Jul 7, 2021 at 15:09
  • The New York Times is also not a scientific report :) The Scotland report (DOI:doi.org/10.1016/S0140-6736(21)01358-1 SARS-CoV-2 Delta VOC in Scotland: demographics, risk of hospital admission, and vaccine effectiveness) results are partially modeled, not taken directly from the data ("no formal significance test to compare the vaccines was done"). I would stick to reporting only information from actual studies and get rid of all the news story references. The answer to your actual title question remains "we don't know".
    – Armand
    Commented Jul 7, 2021 at 20:32
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    If you have a better answer, the post it. I've done my due diligence in aggregating data from a wide variety of sources. Some may be noisy or inaccurate, but others won't be. The idea being that a larger sample size ultimately helps filter out noise and outliers. I'm accepting my answer as the correct answer if no one produces anything better. Commented Jul 7, 2021 at 20:57
  • Sometimes, as in this case, there is no answer. Much misinformation is spread by demanding an answer when there isn't one. You can always ask a different question than the one in your title about the Israeli announcement.
    – Armand
    Commented Jul 8, 2021 at 3:16

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