11 September 2023 

The rapid outbreak of mpox (formerly known as monkeypox) in 2022 likely resulted from high levels of sexual mixing among some gay, bisexual and other men who have sex with men (GBMSM), with the initial downturn in cases probably due to a reduction in sexual contacts among these men, according to new research led by the National Institute for Health and Care Research (NIHR) Health Protection Research Unit (HPRU) in Behavioural Science and Evaluation. The HPRU is a partnership between the University of Bristol and the UK Health Security Agency (UKHSA).

The paper, published in The Lancet Infectious Diseases today [11 September], also found that the scale-up in vaccination did not contribute much to the initial downturn in cases, although the very small number of cases in 2023 is likely due to sufficiently high vaccination coverage to prevent a widespread resurgence in cases.

The 2022 global mpox outbreak spread among GBMSM, with the World Health Organisation (WHO) declaring the outbreak a Public Health Emergency of International Concern on 23 July 2022.

The paper describes the results of mathematical modelling, funded by the NIHR, to understand the epidemiological characteristics of the mpox outbreak in England, and what contributed to the outbreak ending, to help prevent and control future outbreaks.

The authors used anonymous data from over 3,000 mpox cases in England between 1 May and 16 November 2022.  Using multiple data sources from these cases as well as data on the number of vaccinations provided the authors carried out a number of detailed analyses of the English outbreak.

Mathematical modelling undertaken by the team estimated that most infections were among a small subgroup of GBMSM at higher risk for mpox, with the outbreak’s initial decline probably due to a 45% reduction in the sexual partner rate and a 20% decrease in the time that people were infectious, resulting from improvements in diagnosis and isolation. These changes, partially resulting from UKHSA, NHS and community partners’ extensive health promotion and awareness raising, prevented 165,896 infections by
16 November 2022.

Vaccination of GBMSM at high risk of mpox, which started on 27 June 2022, is estimated to have had less impact on the outbreak’s initial decline, but may have prevented a resurgence in cases from January 2023.

Peter Vickerman, Professor of Infectious Disease Modelling from the Bristol Medical School’s Population Health Sciences and NIHR Health Protection Research Unit (NIHR HPRU), and the paper’s corresponding author, said:

“Reductions in sexual risk behaviour among GBMSM at high risk of mpox, together with prompt diagnosis and isolation of people with mpox, can reduce the spread of an mpox outbreak to low levels.”

Sema Mandal, Medical Consultant Epidemiologist in the UKHSA Blood Safety, Hepatitis, STI and HIV Division, and the UKHSA’s senior author added

“While the initial decline in cases was due to less sexual mixing, vaccination amongst GBMSM at highest risk of mpox has contributed to sustained low numbers of mpox cases and likely prevented a resurgence. Vaccination was key to reducing the severity of symptoms and preventing further transmission. It’s important to remember that while mpox case numbers across England remain very low, we are not complacent and stand ready to respond to any significant rise in cases.”

The authors highlight the need for more evidence on the protection provided by vaccination in terms of the strength of effect and duration of protection. This is especially important because of emerging evidence that new infections have been occurring among people that have been fully vaccinated.

The study was funded by Bristol and UKHSA’s NIHR Health Protection Research Unit (HPRU) in Behavioural Science and Evaluation (NIHR200877) in collaboration with University College London and UKHSA’s HPRU in Blood Borne and Sexually Transmitted Infections.

The views expressed are those of the author and not necessarily those of the NIHR, the Department of Health and Social Care, or UKHSA.


Mathematical modelling of the transmission dynamics and impact of control measures in the 2022 outbreak of mpox among gay, bisexual and other men who have sex with men in England‘ by Xu-Sheng Zhang, Sema Mandal, Hamish Mohammed, Professor Peter Vickerman et al. in The Lancet Infectious Diseases

Further information 

More information about mpox is available on the NHS website.

About the NIHR Health Protection Research Unit [HPRU] in Behavioural Science and Evaluation at the University of Bristol
The NIHR HPRU in Behavioural Science and Evaluation at University of Bristol is one of 14 HPRUs across England, part of a £58.7 million investment by the NIHR to protect the health of the nation.

The NIHR HPRU in Behavioural Science and Evaluation is a partnership between the UK Health Security Agency (UKHSA) and University of Bristol, in collaboration with MRC Biostatistics Research Unit at the University of Cambridge and University of the West of England.

Each NIHR HPRU undertakes high quality research that is used by UKHSA to keep the public safe from current and emerging public health threats.

About the National Institute for Health and Care Research (NIHR)
The mission of the National Institute for Health and Care Research (NIHR) is to improve the health and wealth of the nation through research. We do this by:

  • Funding high quality, timely research that benefits the NHS, public health and social care;
  • Investing in world-class expertise, facilities and a skilled delivery workforce to translate discoveries into improved treatments and services;
  • Partnering with patients, service users, carers and communities, improving the relevance, quality and impact of our research;
  • Attracting, training and supporting the best researchers to tackle complex health and social care challenges;
  • Collaborating with other public funders, charities and industry to help shape a cohesive and globally competitive research system;
  • Funding applied global health research and training to meet the needs of the poorest people in low and middle income countries.

NIHR is funded by the Department of Health and Social Care. Its work in low and middle income countries is principally funded through UK Aid from the UK government.

The NIHR is the research partner of the NHS, public health and social care.

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