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The increase in cases of strep A could be linked to lower immunity in the aftermath of COVID lockdowns, a senior health official has said.
Dr Susan Hopkins, chief medical adviser at the UK Health Security Agency (UKHSA), said the agency was exploring whether decreased immunity was a factor in the unusual amount of cases but that it was too soon to draw any conclusions.
Seven children are reported to have died from invasive Group A streptococcal disease (iGAS) in the UK since September.
The number of cases of iGAS has risen this year, the UKHSA said, while there have also been "marked increases" in scarlet fever, which can be caused by strep A.
Strep A is a bacteria that is sometimes found in the throat or on the skin, and can be caught through close contact and from coughs and sneezes.
Usually, symptoms are mild such as a sore throat or a skin infection that may be treated with antibiotics, but it can cause scarlet fever, a disease that can come with a rash and a high temperature, sore throat and swollen neck glands. Scarlet fever also requires antibiotics.
Watch: UKHSA explains what Strep A is and what its symptoms are
In very rare cases, strep A may get into the bloodstream and cause iGAS, which can be fatal. Symptoms include a high temperature of above 38C and severe muscle aches.
The UKHSA has told GPs to be vigilant for severe strep A infections and to refer affected children to hospital immediately.
It said GPs should set a "low threshold" for sending children potentially suffering from strep A to hospital and prescribing them antibiotics.
The UKHSA said there were 2.3 cases per 100,000 children aged one to four years old of iGAS this year, compared to an average of 0.5 in the pre-pandemic seasons (2017 to 2019).
There have been 1.1 cases per 100,000 children aged five to nine this year compared to the pre-pandemic average of 0.3 (2017 to 2019) at the same time of the year.
In the week of 14 to 20 November, there were 851 cases of scarlet fever reported, compared to an average of 186 for the same period in previous years.
Read more: What is Strep A and what are the symptoms?
Dr Hopkins told BBC Radio 4's Today programme on Monday: "Firstly, I think that we’re seeing a lot of viral infections circulate at the moment and these bacterial infections can come as an addition on top.
"Secondly, we’re back to normal social mixing and the patterns of diseases that we’re seeing in the last number of months are out of sync with the normal seasons as people mix back to normal and move around and pass infections on.
“We also need to recognise that the measures that we’ve taken for the last couple of years to reduce COVID circulating will also reduce other infections circulating.
"And so that means that, as things get back to normal, these traditional infections that we’ve seen for many years are circulating at great levels."
Asked if reduced immunity following the end of COVID lockdown measures might explain the rise in strep A cases, Dr Hopkins said: “That’s one of the potential areas that we’re exploring.
"We expect that a certain amount of children will have these infections each year and, therefore, they will have a level of immunity. So we’re seeing more now than we have seen for the last two years where there were very, very low amounts of infection seen.”
Are outbreaks linked to COVID lockdowns?
Health experts are not ruling out a link between the rise in strep A and reduced immunity due to lockdown.
Shiranee Sriskandan, professor of infectious diseases at Imperial College London, said there is a "potential" link.
She added: "Children normally catch scarlet fever in their first year at school, if at all. We know that scarlet fever rates plummeted during 2020 to 2021.
"We therefore think that school aged children may not have built up immunity to strep A, and so we now have a much larger cohort of non-immune children where strep A can circulate and cause infection."
Dr Simon Clarke, a microbiologist at the University of Reading, said: “It strikes me that as we are seeing with flu at the moment, lack of mixing in kids may have caused a drop in population wide immunity that could increase transmission, particularly in school age children.”
Dr Michael Head, senior research fellow in global health at the University of Southampton, said: “From the UK public health surveillance, there are increased numbers of cases of scarlet fever, and also invasive group A streptococcal infection, that would normally be seen at this time of year.
“However, reporting across the season is lower than the equivalent five-year average, and during 2017/18, cases across the year were at much higher levels than are being observed this year.
“After the pandemic response over the last two and a half years, it might be that we’re not yet back in line with usual seasonal expectations, and we do usually observe natural fluctuations in disease patterns.”
Watch: What is strep A infection?