The rapid increase in measles means children are being forced to self-isolate for 21 days if they have not been vaccinated against the disease.
Councils warned parents after it emerged there could be 160,000 cases in the capital alone.
Health officials say low uptake of MMR (measles, mumps and rubella) shots have increased the risk of the potentially fatal infection.
Parents in London and surrounding counties were told that if a student is infected with measles, classmates and siblings who have not had both doses of MMR could be sent home and asked to self-isolate for three weeks.
It’s another blow to children who have missed months of classes during the pandemic.
Councils warned parents after it emerged there could be 160,000 cases in the capital alone
Official data for October to December 2022 shows the areas most vulnerable to a possible measles outbreak, with more than a quarter of children in London missing out on the MMR jab
Data from NHS England released earlier this year shows that uptake of the MMR vaccine fell to just 88.6 per cent for one dose among two-year-olds, and to 85.5 per cent for both jabs among five-year-olds.
Barnet Council in London was among those to send letters to parents warning that their child could face a three-week isolation period from school if they are found to be in close contact with a measles patient.
‘We are currently seeing an increase in measles cases in neighboring London boroughs, so now is a good time to check your child’s MMR vaccination – which will protect your child not only against measles, but also against the mumps and rubella – up to date is’ it reads.
‘Any child identified as a close contact of a measles case without satisfactory vaccination status may be asked to self-isolate for up to 21 days.
‘Children who have been vaccinated do not need to be excluded from school or childcare.’
The Telegraph reported that a similar letter had been sent by neighboring Haringey Council.
The 21-day isolation period is based on guidance published in 2019 by the UK Health Security Agency’s (UKHSA) predecessor Public Health England.
UKHSA told MailOnline that if a case of measles is discovered in a child health team, they will work with schools to provide advice on next steps for children who have not had both MMR jabs.
This may include the offer of MMR vaccination, the provision of measles preventive medicines for close contacts of the child with vulnerable health conditions, and possible exclusion for up to 21 days.
Siblings of an unvaccinated child who has been in close contact with a case of measles may also be asked to self-isolate.
Dr. Vanessa Saliba, epidemiologist at UKHSA, said: ‘Measles spreads very easily and can be serious, especially in vulnerable people such as babies, pregnant women and people with weakened immune systems.
‘Cases are advised to stay away from nursery, school or work until at least four days from the time the rash first appears to prevent them from spreading the infection to others.
‘If there are cases of measles or outbreaks in nurseries or schools, the UKHSA Health Protection Team will work with the school and other local partners to assess the situation and provide advice to staff and students.
‘Those who are not up to date on their MMR vaccinations will be asked to urgently catch up to help stop the outbreak and minimize disruption in schools.’
The MMR shot, which provides lifelong protection against the diseases, consists of two doses and is 99 percent effective in preventing measles infection.
In Britain it is given first when a child turns one and then again when they are three years and four months old to provide the best protection.
Measles, which usually causes flu-like symptoms and a rash, can cause very serious and even fatal health complications if the disease spreads to the lungs or brain.
Doctors are increasingly concerned that measles, long kept at bay thanks to these vaccines, could return due to declining uptake.
National uptake for getting both MMR jabs is currently around 85 per cent, meaning around one in six children of school age are not fully protected.
But vaccination rates in London are extremely low: just 74 percent, meaning one in four children could be at risk.
Both national and regional figures are well below the target needed for herd immunity, with diseases at low risk of spreading through the population (95 percent).
Acceptance of the MMR jab collapsed after research by the now discredited doctor Andrew Wakefield, who wrongly linked the jabs to autism.
Uptake of MMR in England was around 91 percent before the publication of Wakefield’s research, but fell to 80 percent in the aftermath.
Although numbers have recovered slightly, thanks to concentrated efforts by health officials, a rise in anti-vax sentiment during the Covid pandemic is believed to have contributed to some parents choosing not to get their children jabbed.
Meanwhile, it has emerged that a total of 42 cases of the super-mutated Pirola Covid variant have been seen in Britain, up from 36 last week.