A GIANT SIGN at a London hospital warns visitors to go no further if they have a spotty rash and fever. There are outbreaks of measles across the capital, it explains. Last year the World Health Organisation declared measles “eliminated” from Britain, meaning there was no native virus in continuous circulation. The designation stands because public-health authorities stop measles from spreading every time it is imported by someone infected overseas. But their work is becoming harder.

The nearly 1,000 measles cases so far this year are about as many as in the past three years combined. Outbreaks in other European countries are spilling over, just when England’s child-vaccination rate for measles has fallen for four years in a row. At 87%, it is short of the 95% needed to prevent the spread of the virus. Though measles is seen as a childhood illness, most recent cases have been among adults. Rates of immunity are particularly low among those born in the late 1990s, when a now-discredited study by Andrew Wakefield, a doctor who was later struck off, linked one common measles vaccine to autism, scaring parents off immunising their children.

A newer problem is the low take-up of vaccination among immigrants (see chart). Some do not register with a doctor in Britain and seek treatment in their home country when they fall ill. Roma, an ostracised minority in continental Europe, distrust the authorities. And doctors struggle to communicate with mothers who speak little English. The registration form in one surgery in east London lists two dozen nationalities. A further complication in the capital is that its residents move house frequently, which makes it hard to track children due for vaccination. (This may also cause some undercounting of immunised children in official statistics.)

Stopping an outbreak of the illness, 30% of whose victims end up in hospital, involves a lot of detective work. When someone with measles shows up at a clinic, a race against the clock begins. Public-health officials scramble to find anyone who was near the patient in the four days before symptoms appeared (the infectious phase) and check whether they are immune. Anyone who is not must be convinced to act quickly. If taken within 72 hours of contact with the virus, the measles vaccine can prevent the onset of the disease. For those who cannot be vaccinated, such as cancer patients and pregnant women, a dose of antibodies within six days of exposure can prevent measles or reduce its severity.