Study backs theory that TB jab could guard against asthma

A Sydney epidemiological study has given further weight to a theory that exposure to non-viral respiratory infections in childhood may protect against asthma later in life.

Researchers at the Woolcock Medical Research Institute and the Centenary Institute of Cancer Medicine and Cell Biology found that children in allergy-prone families were at significantly lower risk of becoming asthmatics if they had been vaccinated against TB in infancy.

"It's another piece of evidence that microbial exposure in the neonatal period, while the immune system is still developing, has consequences beyond childhood," said Dr Guy Marks, head of epidemiology at the Woolcock Institute.

But the study, involving 751 Sydney children born to immigrant mothers from south-east Asian countries, has created a new puzzle.

Around 25 per cent of Australian children suffer from asthma, and many suffer the added misery of hayfever and the itching skin condition eczema. The syndrome has been assumed to involve an inherited susceptibility to allergies.

While the BCG (Bacille Calmette Guerin) TB vaccine appears to protect allergy-prone children against asthma, the Sydney researchers found it had no effect on their susceptibility to hayfever and eczema.

The Sydney finding contrasts with an earlier study in Guinea-Bissau in Africa, which found that TB vaccination protected against all three disorders.

"The results of our study don't neatly fit that hypothesis," Marks said.

Marks says that unlike some other states, including Victoria, there has been no policy of vaccinating all children in NSW against TB.

This provided a unique opportunity to study vaccinated and unvaccinated children from two different regions of the city, both of which have a high prevalence of one of the major triggers for asthma attacks -- tiny particles of dust-mite excrement in household dust.

From birth registries, the researchers selected 309 BCG-vaccinated children aged 7-11 from one region of Sydney, and a control group of 442 unvaccinated children from another region.

Both vaccinated and non-vaccinated children shared the same rate of allergic sensitisation, but among families in the study group with a history of hayfever or eczema, the rate of asthma was only 46 per cent of that in the control group.

Previous studies and observations have suggested that children born into large families, or who attend childcare centres from an early age, where they are exposed to numerous respiratory infections, are relatively protected against asthma, allergy and eczema.

A European study found that children who live on livestock farms early in life are also protected against asthma, and Marks said there was similar evidence from a study of children living in rural areas of NSW.

Researchers have proposed that the protection arises from a phenomenon known as "immune deviation" -- early exposure to respiratory microbes induces the establishment of clones of a special class of T-cells with a 'memory' for the microbial antigens they have been exposed to -- and these clones secrete cell-signalling molecules that suppress the allergy response.

According to their research paper, published in last month's Journal of Allergy and Clinical Immunology, mycobacterial infections like TB are potent stimulators of the T-1 type immune response. The BCG vaccine, which mimics a TB infection, induces the same strong response.

Marks said it was not clear whether a specific response against the antigens in the BCG vaccine is protective against asthma in allergy-susceptible individuals, or whether it switched the immune system's development into a pathway that suppresses the allergy mechanism.

He said the new study raised the possibility that it might be possible to vaccinate against asthma in childhood, "But one message we want to give is that we are not recommending BCG vaccination in childhood.

"It's not particularly effective against TB, but it does prevent young children from developing severe, life-threatening forms of TB."

Nor would the results justify any relaxation of house hygiene, such as the use of antibacterial agents.

"We have to be very careful out undoing any gains from hygiene measures," Dr Marks said. "The long-term aim of this sort of work is to try to work out what exactly it is about the lack of hygiene that confers protection against asthma and, possibly, allergy.

"Is it possible, for example, to allow exposure to that element without exposing children to any additional risks?"