Video report by ITV News Health Editor Rachel Younger
Medical evidence needs to be explored "carefully" before the UK's meningitis B vaccination programme is extended, David Cameron has said.
The Prime Minister said he was proud Britain had already taken some "very important steps" in being the first country to introduce a vaccination programme against the potentially fatal infection.
Speaking at Prime Minister's Questions, Mr Cameron extended his condolences to the parents of two-year-old Faye Burdett, who died from meningitis B on Valentine's Day.
Cameron said the government and expert advisors would need to "look at all the evidence carefully" before making any decision.
His comments came after the Department of Health (DoH) announced that a proposal to vaccinate all children up to 11 would not be cost effective.
The DoH's announcement came in the wake of more than 800,000 people signing a petition calling on the Government for the vaccine to be given to children at least up to 11.
Currently a vaccine to protect against meningitis B is available on the NHS for babies aged two months, followed by a second dose at four months and a booster at 12 months.
The petition was promoted by several high-profile cases involving the infection, that can kill its victims or leave them in need of life-changing amputations.
But Mr Cameron insisted the Joint Committee on Vaccination and Immunisation believed infants of five months are those most at risk.
Pictures of Faye from Maidstone, Kent, prompted a surge in support for the cause and the petition received a further boost when England rugby hero Matt Dawson revealed his two-year-old son, Sami, was recovering after contracting meningitis C.
"MenB vaccine is offered to infants, free on the NHS, at 2 months with further doses at 4 and 12 months. The programme, as advised by independent experts, offers protection to those at highest risk," the Department of Health said in a statement responding to the petition.
It said immunisation programmes are introduced on the advice of an independent expert body - the Joint Committee on Vaccination and Immunisation (JCVI).
The JCVI had considered older age groups but decided the priority should be the implementation of the primary immunisation programme for infants.
They also considered a programme for adolescents but advised that further research was needed. Preparatory research has been commissioned and is underway, the Department of Health said.
Responding to the petition in a statement on the Government website, the Department of Health said the NHS budget "is a finite resource" and that it was essential recommendations were underpinned by evidence of cost-effectiveness.
With this programme, our priority is to protect those children most at risk of MenB, in line with JCVI’s recommendation.