Twelve people have died in the Essex area and 20 more have been infected by the spread of a rare invasive bacterial infection, Public Health England has said.

A total of 32 people have been confirmed to have contracted invasive group A streptococcus infection (iGAS). Strep A can be carried harmlessly on the skin but can be lethal if it enters the bloodstream. People whose immune systems are compromised by illness or old age are particularly at risk.

Those infected are elderly people receiving treatment for wounds in care homes and in their own homes, according to the NHS Mid Essex. The cases are spread across the Braintree, Chelmsford city and Maldon districts.

Healthy people can develop a relatively mild case of the infection such as “strep throat” that they can shake off, but could infect others through coughing or sneezing without realising the potential seriousness of the bug.

Rachel Hearn, the director of nursing and quality at the Mid Essex clinical commissioning group (CCG), said: “Our thoughts are with the families of those patients who have died. The NHS in Essex is working closely with Public Health England and other partners to manage this local incident, and extra infection control measures have been put in place to prevent the infection spreading in the area.

“The risk of contracting iGAS is very low for the vast majority of people and treatment with antibiotics is very effective if started early. We will continue to work with our partners in Public Health England (PHE) to investigate how this outbreak occurred and take every possible step to ensure our local community is protected.”

NHS Mid Essex CCG said an incident management team had been set up, which it was leading with input from Provide Community Interest Company and PHE, and support from NHS England and NHS Improvement.

Provide is a local social enterprise company that offers health and social care services in community homes and hospitals as well as individuals’ homes.

Group A strep becomes potentially lethal when it triggers sepsis, toxic shock and necrotising fasciitis.

Necrotising fasciitis is very serious, beginning with a red or swollen patch of skin that spreads very rapidly, pain and fever. Patients need rapid intravenous antibiotics and potentially surgery if they are to survive. It is hard to diagnose: by the time a biopsy has been analysed, it may be too late. It can lead to sepsis, shock and organ failure. One in three patients die, even with treatment.

Cases of iGAS have to be notified to PHE. In its latest report to May this year, it registered 1,500 cases notified from September to April in England, which was lower than the previous year but 8% higher than the average for the previous five years.

“While the number of iGAS cases notified through routine laboratory surveillance in England is lower than seen last season, notifications remain at the high end of what would be expected,” said the PHE report.

“Clinicians, microbiologists and HPTs [health protection teams] should continue to be mindful of potential increases in invasive disease and maintain a high index of suspicion in relevant patients as early recognition and prompt initiation of specific and supportive therapy for patients with iGAS infection can be life-saving.”

Dr Jorg Hoffmann, the deputy director of health protection for PHE East of England, said: “All those affected are in vulnerable groups, which puts them at higher risk for what is normally a rarer form of group A streptococcal infection. I would like to emphasise that the risk of contracting iGAS is very low for healthy people and treatment with antibiotics is very effective, if started early enough.

“This is still an evolving situation and colleagues in the provider organisation, the CCG, NHSE/I [NHS England and NHS Improvement] and PHE are working very hard to contain it.”