At a No 10 news conference on Wednesday, February 3, he said suggestions all adults in the UK could get their first dose by the end of May and the second by the end of August were “very optimistic”. Boris Johnson has hailed the “colossal” effort by health workers in getting a first jab to more than 10 million people – almost a fifth of the adult population – across the four nations. However, England’s deputy chief medical officer Professor Jonathan Van-Tam, who is the senior responsible officer for the new study, said that being able to mix vaccines would give them greater flexibility in future.