BBC News By Rachel Schraer image copyrightGetty Images 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?