Fifteen children under the age of 15 have died from Strep A across Britain this winter.
New data from the UK Health Security Agency (UKHSA) shows 13 children under 15 have died in England since September.
Two other deaths of children have been recorded in Belfast and Wales, taking the UK total to 15.
The condition can normally be treated with antibiotics but symptoms, such as a fever and sore throat, can be mistaken for other viruses.
But experts fear a lack of immunity after Covid-19 lockdowns and social distancing could be to blame.
Group A strep bacteria can cause many different infections, ranging from minor illnesses to deadly diseases.
Illnesses caused by Strep A include skin infection impetigo, scarlet fever and strep throat.
While the vast majority of infections are relatively mild, sometimes the bacteria cause a life-threatening illness called invasive Group A Streptococcal disease.
The UKHSA has said there is no current evidence that a new strain is circulating and the rise in cases is most likely due to high amounts of circulating bacteria and increased social mixing.
Since September, the UKHSA said there have been 652 reports of invasive Strep disease, higher than at the same points over the last five years.
So far this season, there have been 85 cases in children aged one to four, compared to 194 cases in that age group across the whole of the last high season in 2017/2018.
There have also been 60 cases in children aged five to nine.
Since September, 60 deaths have been reported across all age groups in England.
Dr Colin Brown, deputy director of the UKHSA, said: "Scarlet fever and 'strep throat' are common childhood illnesses that can be treated easily with antibiotics.
"Please visit NHS.uk, contact 111 online or your GP surgery if your child has symptoms of this infection so they can be assessed for treatment.
"Very rarely, the bacteria can get into the bloodstream and cause more serious illness called invasive Group A strep.
"We know that this is concerning for parents, but I want to stress that while we are seeing an increase in cases in children, this remains very uncommon.
"There a lot of winter bugs circulating that can make your child feel unwell, that mostly aren't cause for alarm.
"However, make sure you talk to a health professional if your child is getting worse after a bout of scarlet fever, a sore throat or respiratory infection - look out for signs such as a fever that won't go down, dehydration, extreme tiredness and difficulty breathing."
It comes as pharmacists continue to use Twitter to complain of shortages in access to antibiotics, including the liquid version of penicillin, which is often given to children.
Health Secretary Steve Barclay insisted on Wednesday that checks within the Department of Health have not revealed an issue with the supply of the medicines.
However, the National Pharmacy Association has pointed to "blips" in the supply chain of liquid penicillin, while the Association of Independent Multiple Pharmacies said pharmacists across the country were struggling to source all they need.
Stella-Lily McCorkindale
Stella-Lily McCorkindale, five, died from Strep A. Her devastated dad Robert said he brought her to A&E three times before she was admitted to the ICU where she later died.
Mr McCorkindale, from Belfast, Northern Ireland, said his "amazing" daughter started feeling unwell on the weekend of November 26 with a cold and temperature.
He said he brought her to the hospital three times in three days before she was admitted, but by then it was too late. Stella-Lily tragically died today as the family is now hoping to raise funds for her funeral.
A review is being conducted into Stella-Lilly's care and the hospital has offered to meet with her family.
Mr McCorkindale told the Mirror: "I think she would have had two extra days of fighting.
"They should have tested her for Strep A on the Monday [November 28], by the time they induced her Stella had given up."
Muhammad Ibrahim Ali
Muhammad Ibrahim Ali, four, died at his home in High Wycombe on November 14 after experiencing a cardiac arrest, his heartbroken mother said.
Ibrahim's headteacher at Oakridge School, Buckinghamshire, said he was a friendly boy who loved being outdoors.
According to Stuart Cook, the school has been working closely with the Health Security Agency and classrooms have been deep cleaned.
Mr Cook said: “Ibrahim was a friendly boy who loved coming to Oakridge School. He had lots of energy and was always active. He particularly loved being outdoors in Forest School.
“Ibrahim was kind and loved to help his friends. He was constantly smiling.
“We are one big family at Oakridge and will miss him terribly. Everyone has been supporting each other as a result of this tragedy.
“We have been touched by the response from the Oakridge School Community who have shared their condolences and laid flowers.
“They have also raised over £4,000 for charity and for a memorial at the school, so that Ibrahim will always have a place here.
“The children have had some lovely thoughts about how we can remember and celebrate Ibrahim’s time here at Oakridge. We will miss him dearly.”
Hanna Roap
Little Hanna Roap died shortly after falling ill with the disease.
The girl from Penarth, south of Cardiff, was given steroids after waking up coughing on November 25 and then she died within 12 hours, it is reported.
Her father, Abul Roap, 37, told The Telegraph: “I took her home from the doctors and gave her the medication. She went to sleep at 4pm and never woke up.
“She stopped breathing at 8pm but we were not immediately aware because she was sleeping.
“I did CPR. I tried to revive her but it didn’t work. Paramedics arrived and continued the CPR but it was too late.
“She did not get the right medication, if she had been given antibiotics it could have been potentially a different story.”
Four other victims
A six-year-old pupil who attended Ashford Church of England Primary School in Ashford, Surrey, also died.
A Year 8 pupil from private Colfe's School in Lewisham, southeast London, has died after contracting Strep A.
Earlier this week, it was announced that another child tragically passed sway in Hampshire after being with an invasive Group A Streptococcal (iGAS) infection.
They attended Morelands Primary School in Crookhorn Lane, in Waterlooville.
