A shot in time


By Nell Boyce VACCINATING children against diseases that normally spread in synchronised waves can make outbreaks become more sporadic, according to scientists in Britain. They warn that this effect should be taken into account, as diseases that break out in irregular patterns are hard to eradicate. Pejman Rohani and his colleagues at the University of Cambridge made this discovery after collecting information on measles and whooping cough outbreaks in 60 cities, towns and villages across England and Wales between 1944 and 1994. In Britain, mass vaccination programmes began in 1968 for measles and in 1957 for whooping cough. When the researchers examined records from before and after immunisation began, they realised that the two diseases showed opposite patterns. Before vaccination, measles broke out in periodic, synchronised waves all across England and Wales. Every two years, most cities would simultaneously experience a measles outbreak. After immunisation, the cities had fewer outbreaks, which appeared irregularly and not in all cities at once. But whooping cough showed the opposite pattern. Before vaccination, individual cities had their own epidemics, which sometimes occurred every two years and sometimes every year. After vaccination programmes began, however, cities had simultaneous outbreaks every three and a half years. “To find the reverse patterns is very exciting,” says Rohani. He thinks the reason for the difference is that people with measles and whooping cough are infectious for different lengths of time, with various consequences. For instance, children with measles are infectious for about five days, so school holidays tend to quash outbreaks, whereas infections spread quickly during term time. This helps synchronise the disease. Vaccinations tend to randomise this pattern. But kids with whooping cough are infectious for around two weeks, so synchrony doesn’t arise naturally. One reason vaccination makes outbreaks synchronise might be that children are older and travel around more by the time they contract the disease. “This study tells us that details matter,” says Rohani. The experiment has intrigued scientists interested in disease eradication. While vaccination clearly reduces disease outbreaks, eliminating a disease becomes more difficult if cities have outbreaks at different times. This study suggests that vaccination might in some cases make a disease more difficult to eradicate, says Ottar Bjørnstad of the National Center for Ecological Analysis and Synthesis in Santa Barbara, California. Rohani speculates that giving vaccines in pulses rather than all at once might allow diseases such as measles to remain synchronised, improving the chances for eradication. But scientists will need to look at each disease individually. “We should exercise caution,
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