London School of Hygiene & Tropical Medicine

A study, newly published in The Lancet Regional Health – Africa, led by researchers at the London School of Hygiene & Tropical Medicine (LSHTM), the College of Medicine and Allied Health Sciences (COMAHS) of the University of Sierra Leone, and Johnson & Johnson Innovative Medicine, has shown that the two-dose Ebola vaccination course (Ad26.ZEBOV, MVA-BN-Filo) causes immune responses (the body’s response to infection) against the Ebola virus for several years, which can be further enhanced by booster vaccination.

Following participants for up to five years

The study recruited adults and children in Kambia District, Sierra Leone, who had previously participated in the EBOVAC-Salone Ebola vaccine trial conducted in the aftermath of the 2014–16 West African Ebola epidemic. In this extension study, researchers followed up adults for up to five years and children for up to four years after their first vaccine dose to better understand long-term safety and immune responses. A total of 652 participants — including 265 adults and 387 children — were enrolled in the follow-up study.

The study recruited adults and children in Kambia District, Sierra Leone, who had previously participated in the EBOVAC-Salone Ebola vaccine trial conducted in the aftermath of the 2014–16 West African Ebola epidemic. In this extension study, researchers followed up adults for up to five years and children for up to four years after their first vaccine dose to better understand long-term safety and immune responses. A total of 652 participants — including 265 adults and 387 children — were enrolled in the follow-up study.

The results were reassuring. Over the long-term follow-up period, serious adverse events — such as hospitalisation, life-threatening conditions, or death — were rare and none were considered related to receiving the Ebola vaccine doses.

The study also measured immune responses to the vaccine over time — an indicator of how the body responds to vaccination. Five years after vaccination, 61% of the adults were still classified as vaccine responders, while 66–91% of children remained vaccine responders four years after vaccination, with the strongest immune responses observed in younger children. Although the level of response required for protection against Ebola virus disease is not yet fully understood, these findings are promising because they suggest that vaccine-induced immune responses can persist for several years.

The role of booster doses

The study also provided valuable evidence on booster vaccination.

Adults who received an additional Ad26.ZEBOV booster dose two years after their initial vaccination had antibody levels eight times higher than those who did not receive a booster.

These findings suggest that booster doses may play an important role in sustaining long-term immunity in populations at risk of Ebola virus exposure, such as healthcare workers or communities living in regions where outbreaks occur.

Dr Daniela Manno, clinical epidemiologist at the London School of Hygiene & Tropical Medicine and lead author of the study, said:

"Our findings show that this vaccine regimen causes immune responses that last for several years, and that booster doses may further strengthen long-term antibody presence in the body. Booster vaccination could be considered in the years following the primary regimen or at the start of an outbreak, when rapid protection is needed. This is especially important as Ebola outbreaks are often not recognised early, and initial symptoms can be confused with common illnesses such as malaria. Healthcare workers are therefore among the first to be exposed, and their infection can contribute to transmission within healthcare settings and amplify early spread.”

Why long-term data matters for epidemic response

Long-term studies such as this one contribute to the growing body of evidence supporting vaccination as a key pillar of epidemic preparedness and help public health agencies plan for effective vaccination strategies before and during outbreaks.

Ebola virus disease has caused repeated outbreaks across sub-Saharan Africa, often with devastating consequences for communities and health systems. Vaccines are now an essential tool for outbreak response and are highly successful at helping curb outbreaks. For example, a recent Ebola outbreak was ended in the Democratic Republic of the Congo thanks to the quick use of over 48,000 vaccines which were given to more than 44,400 people including frontline health workers and high-risk contacts.

Despite their immediate impact during ongoing disease outbreaks, little is known about how long protection from the Ebola vaccine lasts, a factor that could impact how authorities plan vaccination strategies for future Ebola outbreaks, particularly how best to protect frontline healthcare workers and other populations at risk, including decisions on the optimal timing of booster doses to help answer this.

As Ebola outbreaks continue to occur across Africa, generating evidence on the durability of vaccine-induced immunity will remain critical for informing vaccination policies and preparedness strategies. Strengthening the evidence base for vaccines against epidemic-prone diseases remains a key priority for global health security because it can support faster and more effective responses to future outbreaks, such as Ebola, and reduce the risk of wider spread at regional and international level.

A long-term collaboration in Sierra Leone

The EBOVAC-Salone programme has been a long-standing collaboration between LSHTM, the College of Medicine and Allied Health Sciences of the University of Sierra Leone, and international partners. The work was made possible through the participation of hundreds of volunteers and the dedication of local clinical, laboratory, and community engagement teams.

Dr Ahmed Dahiru Balami, who coordinated the study in Sierra Leone, highlights the importance of this long-term commitment:

"Sustained follow-up studies like this are essential to understanding how vaccines perform in real-world settings over time. The collaboration with communities and local health teams in Sierra Leone has been fundamental to generating this evidence."

Looking ahead

While the findings provide encouraging evidence of long-term safety and immune responses, important questions remain. Future studies will be needed to help determine how long vaccine-induced immunity may persist beyond five years and improve our understanding of the immune responses that best predict protection against Ebola virus disease.

The full study is available in The Lancet Regional Health – Africa