Children infected with HIV at birth are surviving into adolescence, overturning the assumption that virtually all die before the age of five, doctors working in Zimbabwe will reveal this week.

But because the children's growth has been stunted they face particular difficulties as they enter puberty, which are not being tackled.

Doctors working in sub-Saharan Africa have been noticing older children arriving at HIV clinics with the late stage of the disease. But until now, there has been no proof that this reflected more than the survival from birth of a few individuals.

Researchers from the Wellcome Trust set out to study the older children arriving at a clinic in Harare, Zimbabwe, and have demonstrated that the numbers are significant. Half a million babies are estimated to have been infected with HIV at birth or during breastfeeding last year alone. The Zimbabwe study suggests as many as one in four may survive into adolescence.

"The findings are quite extraordinary," said Dr Liz Corbett, a Wellcome Trust senior clinical fellow in tropical medicine from the London School of Hygiene and Tropical Medicine, based in Zimbabwe.

"The phenomenon of long-term survival is poorly recognised and until recently has been almost positively resisted by the international HIV community because of the strongly held assumptions that HIV in late childhood is very unusual, and that survival from birth to adolescence with HIV was so unlikely without treatment as to be negligible. This just doesn't fit with what we see in Zimbabwe and hear from neighbouring countries.

"It is now being realised that these earlier assumptions were wrong and that instead somewhere around one in 10 infected infants - and perhaps even as high as one in four - may survive into late childhood or early adolescence without diagnosis or treatment."

The study was carried out by researchers at the Connaught clinic for children in Harare and is to be published this week in the journal Clinical Infectious Diseases. They assessed 32 children between eight and 19 who had arrived at the clinic with advanced HIV infection and in urgent need of antiretroviral drugs. The median age of their first HIV test was 11, and most had at that point experienced HIV-related symptoms more than three years earlier.

Rashida Ferrand, lead author of the paper, said it was not possible to be 100% sure that the children were infected at birth, but it was likely because they were Aids orphans or their parents had Aids and they had siblings who had died of Aids. "One of the really surprising things is they are surviving without treatment, while being exposed to other infections," she said.

There have also been observations of older children with late stage HIV infection in other countries with long-established epidemics and some UK children who have come from sub-Saharan Africa.

"The accepted view was that the majority would die before the age of five," she said. "Studies did not follow the kids up. They are surviving but they are not surviving asymptomatically. They get Aids or they are living with recurrent infections which has hugely serious implications." In their study, 62% were stunted and all were severely underweight.

One of the main public health messages, she said, was the need to prevent infection from mother to child during birth and breastfeeding.

Mother to child transmission programmes have been slow to get off the ground in sub-Saharan Africa.

The baby can be protected, but relatively few pregnant women with HIV are offered the chance to protect their baby.