The PM says the return of schools in England on Monday is the "first step" towards "normality".
People in the age group are being invited to book their first vaccine from this week.
The existing recommendations are based on out-of-date evidence, the Royal College of Nursing says.
Parents are confident about children returning to school but worry about catching up, says survey.
Bennylyn Burke and two children had been missing from South Gloucestershire since 17 February.