Covid-19 vaccine "cannot be taken for granted", as he ruled out smaller local lockdowns to contain the virus.The Prime Minister told MPs there is a "good chance" of developing a vaccine before sounding a note of caution.He also faced demands from Tory backbenchers to trust Britons to act responsibly and not leave areas in a "permanent state" with restrictions in place.His remarks came during a lengthy Commons grilling after he announced the new three-tier approach to deal with the spread of coronavirus in England.Tory former minister Steve Baker asked: "By when does he expect to have vaccinated the vulnerable population, what is his confidence in that date and why does he have that confidence?"Johnson replied: "Alas, I can't give him a date.