Question: How much, if any, confidence can we have in these new Israeli studies on the Pfizer vaccine, which show that the vaccine may reduce transmission? 

Answer: As of early February, nearly 90% of Israelis age 60+ had received their first dose of Pfizer and more than 40% of Israelis had been vaccinated with at least the first dose.  With relatively quick vaccine roll-out plus robust data collection, Israel offers unique insights.  Here are three recent take-aways:

1. Pfizer’s real-world effectiveness is similar to clinical trial efficacy. Last week, Israel’s largest healthcare provider, Clalit, reported a 94% drop in symptomatic COVID-19 infections among the 600,000 people who received two doses of the Pfizer’s vaccine as compared with 528,000 individuals who did not receive any vaccination, controlling for a host of background factors.  This finding is quite stunning– real-world effectiveness is mirroring clinical trial efficacy.  Based on what was reported from Clalit in the news, the study sounds quite robust and the findings do align with what we saw in the clinical trial.  Unfortunately, the findings are preliminary and I have not been able to locate any pre-publication report.  To answer your question, I think you can put a good degree of faith in the findings, but I wouldn’t be 100% confident (high threshold) until we actually have the report in hand and it has been peer-reviewed.    
2. First dose of Pfizer decreases SARS-CoV-2 viral load.  A new pre-print publication (not yet peer reviewed) from Israel’s Maccabi Healthcare Services shows that those individuals who became infected with SARS-CoV-2 twelve (12) or more days after their first vaccine shot but before their second shot had statistically (and meaningfully) lower viral loads as compared with similar individuals who had not yet received a first shot as well as individuals who became infected within 1-11 days of receiving their first dose.  I found this read quite compelling and the study design strong.  As with all publications, this one will improve with peer-review, but the overall finding seems to be robust even in light of the limitations the authors outline.  This finding is so important because…
3. Lower viral loads likely mean less transmission. Earlier this month, Lancet Infectious Diseases published, Transmission of COVID-19 in 282 clusters in Catalonia, Spain: a cohort study, which thoroughly examined why some people seem to be more contagious than others (e.g. risk of transmission).  Authors conclude, “the viral load of index cases was a leading driver of SARS-CoV-2 transmission. The risk of symptomatic COVID-19 was strongly associated with the viral load of contacts at baseline and shortened the incubation time of COVID-19 in a dose-dependent manner.”  In essence, higher viral loads were found to lead to more transmission.  Higher viral loads were also more likely to result in more symptomatic cases (FIgure 1).

Figure 1. Relationship to Viral Load and Symptomatic Disease (from Lancet)