Children and adults have shown a strong immune response to a possible Ebola vaccine during clinical trials in Sierra Leone.

The two-dose vaccine from Johnson & Johnson was also found to be safe and well tolerated, according to research published in The Lancet Infectious Diseases.

The EBOVAC-Salone study is the first to test the vaccine in a region affected by the 2014-16 Ebola outbreak, the worst on record.

It is also the first to test the vaccine on children – approximately 20% of cases in that outbreak were in those aged under 15, with children under five being at a higher risk of death.

Some 98% of trial participants showed antibody responses 21 days after the second dose of the vaccine, and the immune response lasted for at least two years in adults.

The two-stage trial was run between September 2015 and July 2018.

The first stage looked at the vaccine’s safety and ability to provoke an immune response, with 43 adults receiving the Ad26.ZEBOV vaccine followed by the MVA-BN-Filo vaccine after 56 days.

They were offered a booster dose of A26.ZEBOV two years after the first dose and this produced a strong immune response within seven days.

In the second stage, 400 adults and 576 children were vaccinated with either the Ebola vaccine regimen or a single dose of a meningococcal vaccine followed by placebo on day 57.

The research was a collaboration between the London School of Hygiene & Tropical Medicine (LSHTM) and Sierra Leone’s College of Medicine and Allied Health Sciences (COMAHS).

First author on the paediatric paper, Dr Muhammed Afolabi, assistant professor at LSHTM, said: “This study represents important progress in the development of an Ebola virus disease vaccine regimen for children, and contributes to the public health preparedness and response for Ebola outbreaks.

“Working in tandem with Sierra Leone colleagues and the local communities, this is the first published study to evaluate this two-dose vaccine regime in a randomised controlled trial in children.

“The results show that this vaccine regimen has the potential to save many young lives.”

Professor Deborah Watson-Jones, from LSHTM, said: “The threat of future Ebola virus disease outbreaks is real and it’s important to remember that this disease has definitely not gone away.

“Despite the additional global challenges around COVID-19, we must not slow down efforts to find effective ways of preventing Ebola virus epidemics and, should outbreaks occur, of containing them rapidly. Vaccines have a key role in meeting both of these objectives.”

The vaccine was granted marketing authorisation by the European Commission in July 2020 and Johnson & Johnson said that more than 250,000 people participating in clinical trials and vaccination initiatives have received at least the first dose, including 200,000 who have been fully vaccinated.

Ebola is spread between humans via contact with an ill person’s blood or body fluids or contact with objects that have been contaminated with those fluids.

Those at higher risk include anyone who cares for an Ebola patient and anyone conducting burial rites that involve direct contact with the body of the deceased.

The fatality rate has varied from 25% to 90% in past outbreaks, according to the World Health Organization.