Colonies of Streptococcus pyogenes, a Gram-positive bacterium, growing on blood agar in a petri dish.

Group A streptococcal bacteria (strep A) cause infections that trigger a sore throat — and can lead to scarlet fever or, rarely, more serious conditions and death.Credit: CNRI/Science Photo Library

As the days lengthen and temperatures rise each spring, British paediatricians know what to expect: an increase in group A streptococcal infections that should tail off by the summer. But an off-season outbreak of the bacterial infections this year has jumbled expectations, made scores of people ill and killed 13 children under the age of 15 in England since September.

“To my knowledge, we’ve never seen a peak like this at this time of year, at least not for decades,” says microbiologist Shiranee Sriskandan at Imperial College London.

One theory is that lack of exposure to group A Streptococcus (strep A) during lockdowns at the height of the COVID-19 pandemic means that young children lack immunity against the bacteria. But it is too early to say for certain if this is behind the strep A surge, says molecular microbiologist Claire Turner at the University of Sheffield, UK.

“There are a lot of things that seem to be a bit strange happening after the lockdowns,” she says. “But it’s hard to say whether that’s causing the surge right now, especially given that we have had surges prior to the pandemic.”

Scarlet fever

A strep A infection most often presents as a mild sore throat (often called strep throat) or skin infection. Occasionally, this can lead to a condition called scarlet fever, which typically affects young schoolchildren and is characterized by a sore throat, high fever and rough skin rash. If caught in time, strep A infections can usually be treated with antibiotics.

In rare instances, strep A can lead to a more serious condition called invasive group A streptococcal infection, particularly in people with compromised immune systems. This can cause conditions including meningitis, toxic shock syndrome and necrotizing fasciitis, a swiftly progressing infection of the tissue under the skin that is sometimes called flesh-eating disease.

In England, scarlet fever is carefully tracked, and physicians are required to report cases to health authorities. Between mid-September and mid-November, there were 4,622 notifications of scarlet fever in England, compared with an average of 1,294 in the same period over the past five years. (The other nations of the United Kingdom track health statistics separately.)

Those numbers don’t show the true extent of strep A infections in the country, says Sriskandan. “Scarlet fever is the tip of the strep-throat iceberg,” she says. “It’s a very visible marker of what’s going on in the community.”

Almighty surge

A similar pattern of off-season strep A infections is being observed in other countries, too, says microbiologist Nina van Sorge at the Amsterdam University Medical Center. “It’s definitely not just a UK thing,” she says. “Cases are still remarkably high in the Netherlands.”

England has been experiencing high levels of springtime scarlet fever for years, and in 2018 there was an “almighty surge”, says Sriskandan. Each surge brings an expansion of strains that are fitter than previously dominant versions, and one that became dominant around 2016 happened to be particularly prone to causing serious illness, she says.

In early 2020, UK paediatricians were bracing themselves for a rough strep A season. Then the pandemic swept in, schools closed, people stayed at home and rates of respiratory infections and scarlet fever plummeted.

But 2022 has brought more social mixing — and more scarlet fever. And this year, rates did not fall in the summer as expected. Instead, the number of cases of scarlet fever in England has continued to climb, with brief lulls when schools were closed for holidays.

Are lockdowns to blame?

The theory that the surge can be blamed on a lack of exposure to strep A during the height of the pandemic is under discussion. This gap in exposure has also been blamed for this year’s increased rates of influenza, chicken pox and respiratory syncytial virus (RSV).

Turner and her team are hoping to evaluate samples from this year’s surge using 3D cultures of tonsil cells grown in the laboratory. They hope to learn more about whether this year’s strains differ from previous ones in how they infect the throat.

For now, health officials are trying to raise awareness among the public and physicians about the off-season outbreaks. Serious strep A infections can progress quickly: in the Netherlands, public-health officials have urged physicians to treat suspected cases with antibiotics immediately, rather than wait for more information, says van Sorge.

If the current surge is related to pandemic social distancing, the hope is that cases will return to their normal seasonal patterns in the coming years, once community levels of immunity are bolstered, says van Sorge. “I would sincerely hope that after this surge, kids will have been exposed again, they will build up immunity — and that’s it.”