Israel, one of the top countries when it comes to vaccinating against Covid-19, bought large stocks of the jab in exchange for acting as the world's guinea pig.
And scientists are watching data shared by the country keenly, for signs of how effective the vaccine is when given to a whole population.
So there was understandable concern when the man coordinating Israel's Covid response reportedly suggested a single dose of the Pfizer vaccine might not be as effective as reported.
Thousands of people have tested positive after being jabbed. But are worries premature?
Addressing Prof Nachman Ash's quotes, the Israeli Ministry of Health said: "The comments of the Israeli Covid-19 commissioner regarding the effect of the first dose of the vaccine were out of context and, therefore, inaccurate.
"The commissioner said we have yet to see a decrease in the number of severely ill patients."
The "full protective impact of the vaccine" was expected to be seen soon, a spokesperson added.
After vaccination, the body needs time to recognise the virus's genetic material and mount an immune response - producing the antibodies and T-cells that block the virus from entering cells and killing off those cells that do become infected.
And it takes a minimum of two weeks - but probably more - to really take effect, immunologists including Prof Danny Altmann at Imperial College London, say.
Some of the people who were hospitalised in recent weeks will have received a first dose of the vaccine. But that's not necessarily a sign it hasn't been effective.
Given the time it takes for immunity to build and unvaccinated people to become exposed to the virus, incubate it and develop symptoms, the national data would not be expected to reflect the impact of vaccination for at least a month.
Clalit, Israel's largest healthcare provider, covering most of the population, compared the medical records of 200,000 vaccinated over-60s with the same number belonging to unvaccinated people.
For two weeks after the first dose, both groups tested positive for the virus at roughly the same rate.
But then those vaccinated started to show 33% fewer new infections than the others.
"This is very early protection," said Clalit chief innovation officer, Ran Balicher, who is also a public health professor advising the government.
And "already there is a 33% fall".
Infections continued to decline after that.
But there were too few for Prof Balicher to calculate by what percentage.
The London School of Hygiene and Tropical Medicine's Prof Stephen Evans said: "It is not sensible to compare efficacy derived from an observational study of this type, which is subject to many biases, with the efficacy derived from randomised trials [such as those performed to test the vaccine in the first place]."
But the early evidence does follow roughly the same trend seen in Pfizer's clinical trials, which involved tens of thousands of people.
In those trials, the rate of new infections in the vaccinated and control groups also diverged after two weeks.
And that gap kept widening for more than 100 days after the first dose.
Israel began vaccinating on 19 December, reaching 10% of its population by the end of 2020.
And now, about half a million of its nine million population have received two doses.
Prof Balicher suggests some impact on severe Covid illness across Israel could be expected to be seen later this week.
Pfizer hopes two doses of the vaccine will give up to 95% protection.
But even a much less effective vaccine could make the difference between a survivable illness and a fatal one.
The annual flu vaccine is about 40-60% effective.
And it stops hundreds of thousands of people from dying every year.
"Efficacy against detected Covid-19 may be less important... than efficacy against hospitalisation and death," Prof Evans said.
And more people were likely to be protected from a severity of illness requiring admission to hospital than infection rates alone suggested.
The Clalit data, for example, does not distinguish between mild or symptomless infections and more severe or even fatal disease.
The judgement made in the UK was more lives could be saved by giving a larger group one dose quicker than giving a smaller group the maximum protection conferred by two doses.
And Prof Evans said: "The reports that have come from Israel are insufficient to provide any evidence that the current UK policy in regard to delaying the second dose of vaccines is in any way incorrect."