Lower immunity after lockdown likely impacted outbreak of deadly Strep A infections
Published: 5 November 2026
Researchers studying the winter 2022/2023 outbreak of Group A streptococcus infections in Scotland have found strong evidence that the outbreak was not caused by a new virulent strain of the disease, but was likely the result of pandemic isolation precautions
Researchers studying the winter 2022/2023 outbreak of Group A streptococcus infections in Scotland have found strong evidence that the outbreak was not caused by a new virulent strain of the disease, but was likely the result of pandemic isolation precautions.
The study – published in the Journal of Infection and led by the University of Glasgow in collaboration with the Scottish Microbiology Reference Laboratory and Houston Methodist Hospital – suggests that the surge in Strep A cases was due to lowered immunity levels as a result of less mixing and exposure to bacteria during lockdown precautions, not because a strain of the bacteria had become more dangerous.
Scotland, like many countries, saw a sharp rise in serious Strep A infections in late 2022 and early 2023, when cases rose to four times their normal level in children, having fallen by about 70% during COVID-19 restrictions. The increase infections led to a significant demand for antibiotics and GP or hospital appointments.
While Strep A infections usually cause sore throats and scarlet fever, it occasionally enters the blood, lungs or other parts of the body and becomes life-threatening. The outbreak caused 28 child deaths across the UK four nations over a five-month period. Older adults were also significantly affected.
Scientists proposed three explanations: people had lower immunity after lockdowns, flu and other viruses were paving the way, or a new, more dangerous strain of the bacteria had appeared.
To test these theories, the researchers looked at every serious Strep A infection recorded in Scotland from 2014 to 2024 – a total of 3,408 cases.
The researchers also read the complete genetic code of over 400 samples of the most common strain of Strep A, emm1, which were collected before, during and after the pandemic.
The genetic analysis found no sign of a new or more dangerous strain – bacteria from after the pandemic were essentially the same as those from before it, and there were no new toxin genes or new antibiotic resistance.
Andrew Smith, Professor of Clinical Bacteriology, Consultant Microbiologist and Director of the Scottish Group A Streptococcus Reference Laboratory, said: “The big question in 2022 was whether a new, more dangerous strain of Strep A was behind the surge. To find out, we looked at eleven years of Scottish surveillance data and read the complete genetic code of more than 400 bacterial samples taken from infections before, during and after the pandemic.
“The bacteria causing infections after the pandemic were essentially the same as those circulating before it. The surge was driven by changes in us, not in the bug: lower immunity after lockdowns, combined with flu and RSV returning at the same time. That tells us where to focus protection when infections bounce back.”
The paper, ‘Clinical and population genomic epidemiology of invasive group A streptococcus in Scotland, 2014-2024,’ is published in the Journal of Infection.
Enquiries: ali.howard@glasgow.ac.uk or elizabeth.mcmeekin@glasgow.ac.uk
First published: 5 November 2026