Vaccine rates are overestimated

Vaccination is vital to control disease

Official data on global vaccination rates may exaggerate how many people are immunised against common diseases, say experts.

A team from the World Health Organization examined data on the diphtheria-tetanus-pertussis vaccine.

They found official coverage rates were much higher than those recorded in house-to-house surveys.

Writing in The Lancet, they warn effective public health programmes depend on accurate data.

It's all about locating children who have not been vaccinated Dr Maureen Birmingham

The researchers examined vaccination records for 45 countries in the developing world between 1990 and 2000.

They compared official data submitted by health centres and workers who give out vaccines, with that collected by researchers who asked individual people whether or not they had been vaccinated.

Consistently, the household surveys showed that fewer people had been vaccinated than the numbers suggested by the official returns.

The discrepancy was so large that the researchers calculated that an official vaccination rate as high as 16.3% could in reality mean that no effective vaccinations took place on the ground.

Practical problems

The researchers believe that part of the problem is that data collection systems are not up to the job.

For example, official data often fails to record whether children received their jabs at the proper age, or whether follow-ups were given at the correct intervals.

It may be that lumping "valid" and "invalid" vaccinations together inaccurately inflates the coverage rate.

However, they also warn that it is possible that incentive schemes designed to boost vaccine rates may lead to the figures being inflated.

In addition, they point out that it can be very difficult to keep track of vaccinations delivered outside the public sector.

Although house-to-house surveys appear to give a more accurate picture, they are extremely labour intensive and expensive to carry out, and therefore can only be done on a very limited basis.

Lead researcher Dr Bakhuti Shengelia told BBC News Online: "Reliability of data is important in order to keep a better track of how well we are doing in terms of protecting children from infectious disease."

Dr Maureen Birmingham, of the WHO's vaccine assessment and monitoring team, told BBC News Online that a system of audit had recently been introduced to try to improve the way vaccination programmes were reported.

However, she said it was not fair to suggest that official records always overestimated coverage rates. In fact some countries, such as Kenya, probably underestimated the scope of their vaccination programmes.

"It's all about locating children who have not been vaccinated," she said. "If we don't have accurate local data then we cannot do that."