Maputo — The prevalence in Mozambique of HIV, the virus that causes AIDS, is now 11.5 per cent among people aged between 15 and 49, according to the results of a national survey released by the Ministry of Health on Monday.

At first sight, this looks like a significant drop in the HIV prevalence. In 2008, the prevalence was estimated at 16 per cent and in 2009 at 15 per cent.

But at Monday's release of the latest figures, Health Minister Ivo Garrido warned that the apparent drop of over three per cent in the prevalence figures is a statistical artifact. What has changed is not the number of people carrying HIV, but the way the data are collected.

Up to 2009, the figures were based on blood samples from pregnant women attending around 40 "sentinel" health units across the country. This was by no means a random sample and it was feared that the choice of health units might skew the figures towards the urban population, where infection rates were known to be higher than among rural dwellers.

The latest survey abandons the idea that from an analysis of pregnant women only, reliable inferences can be drawn about the adult population as a whole. Instead the "National Survey on Prevalence, Behavioural Risks and Information on HIV/AIDS" (INSIDA) took as its target group society at large.

The sample consisted of 6,232 households covering every district in Mozambique. The interviews were undertaken between June and October.

The INSIDA figures show that women are much worse hit by the epidemic than men. 13.1 per cent of women aged 15 to 49 tested HIV-positive, as opposed to 9.2 per cent of men.

This difference is repeated across nine of the 11 provinces. Only in the northernmost province of Niassa are more men than women infected (four per cent to three percent), while in Maputo province the rates for the two sexes are the same (20 per cent).

The urban population is at much greater risk than people living in the countryside. The urban prevalence is 15.9 per cent (18.4 per cent for women, and 12.8 per cent for men). In the rural areas, the prevalence is 9.2 per cent (10.7 per cent for women and 7.2 per cent for men).

Infection rates are also higher among better educated people, and among wealthier people - which completely destroys the thesis associated with former South African President Thabo Mbeki that AIDS is a disease of poverty.

Reasonably well-off, literate people in towns are substantially more at risk than illiterate and penniless peasant farmers.

The AIDS crisis is at its worst in the south of the country. In Gaza province, 30 per cent of women tested HIV-positive (but only 17 per cent of men). It is impossible to avoid the conclusion that this alarming figure is connected to Gaza's long history as a pool of migrant labour for the South African gold mines.

The northern provinces of Niassa and Nampula have the lowest prevalence rates, while the central provinces are in between.

Garrido was at pains to point out that, just because these figures are mostly lower than those generated by the epidemiological surveillance rounds at the sentinel health units, nobody should imagine that there had really been a substantial decline in the HIV prevalence.

He believed that the INSIDA results were more reliable than those from the epidemiological surveillance rounds. They were also much more expensive. An epidemiological surveillance round cost about 500,000 US dollars. The INSIDA survey cost six million. Garrido did not expect another such survey until 2014.

The Minister also noted that AIDS has become the main cause of death and hospitalisation among adult Mozambicans. Only among children does malaria outstrip AIDS as a cause of death.

Garrido called for more work to be done on changing the behaviour of young people - particularly young girls. The survey showed that women tend to become infected earlier than men - many before their 20th birthday.

"A vigorous intervention is needed towards girls, and we have to act earlier", said Garrido. "The study shows us that by the time they are 20, it will be too late".