The number of reported cases of scarlet fever almost doubled in 2018 to around 32,000, official figures show.
It follows a series of yearly increases after an unexpected surge in cases in 2014.
Public Health England (PHE) said the suspected 31,904 scarlet fever cases reported by doctors in 2018 was the highest since 1960.
By comparison in 2017, there were 17,813 notified cases. Before 2014 there were only around 2,000 reported cases a year and public health experts are still trying to determine the cause of the increase.
PHE researchers had warned two years ago that the disease seemed to be making a comeback.
In a paper published in The Lancet in 2017, they pointed out that England is the first western hemisphere country to describe an upsurge in scarlet fever incidence after reports in several countries in east Asia.
Professor Shiranee Sriskandan, an infectious disease expert at Imperial College London said several reasons have been proposed for the increasing cases but it was still not entirely clear what was happening.
She said: ‘Scarlet fever is a very visible signal of the amount of group A streptococcal pharyngitis that is circulating.
‘It is believed that fluctuations in rates of scarlet fever are linked to a balance between new strains circulating, and immunity to those strains amongst the main vulnerable population, ie, young children.’
She added that an increase in cases in 2016 had been associated with the emergence of a new strep A clone but it is not known if that clone or other strains were involved in the 2018 increase.
She said: ‘Reassuringly this year [2019] the numbers have fallen, although not back to the levels that were seen six or seven years ago. It’s too soon to know if the decline will continue next year.’
A spokesperson for Public Health England said the most up to date seasonal data for notifications had fallen compared with the past few years.
They pointed out that the data shows that there have been increases across the country and despite some speculation no apparent link with deprivation so far.
A spokesperson for PHE said: ‘We are investigating possible explanations as to why there have been more scarlet fever cases over the last few years, by studying the strains of bacteria causing disease, and studying the spread of infection in different settings and patient groups.’
