Ebola vaccine halves deaths even in people already infected

The Ebola vaccine can protect people from dying of the disease even if they were already infected at the time they were given the jab, according to a new study.

The research, based on data from the 2018–2020 Ebola outbreak in the Democratic Republic of the Congo (DRC), showed that the risk of dying of Ebola was halved among people who had been vaccinated with a single dose of the rVSV-ZEBOV vaccine before they started showing symptoms.

Ebola isn’t the first vaccine to be highly effective post-exposure – so-called ‘therapeutic vaccines’ given after infection rather than before have been used with rabies, measles and smallpox.

Even people who had been vaccinated just a day or two before they got sick were protected, despite the fact that it wouldn’t have been enough time for their immune systems to launch a full-blown response to the vaccine – scientists believe our bodies need about ten days to do this. The death rate among people who had been vaccinated two or fewer days before their symptoms developed was 27%, compared to 56% in those who were unvaccinated.

The study was conducted by researchers at Médecins Sans Frontières, in collaboration with the Institut National de Recherche Biomédicale (INRB) and the Ministry of Health of the Democratic Republic of the Congo. It was published in The Lancet Infectious Diseases.

The researchers noted that people who had been vaccinated had lower virus levels than those who hadn’t been vaccinated, which was likely a key factor in protecting them.

The Merck-manufactured vaccine was given to 40,000 people over the course of the DRC outbreak, and as it is 95% effective, it played a key role in ending the epidemic.

Ebola isn’t the first vaccine to be highly effective post-exposure – so-called ‘therapeutic vaccines’ given after infection rather than before have been used with rabies, measles and smallpox.

Sometimes vaccines are used this way because it is not practical logistically or financially to vaccinate everyone who might be at risk of disease – this is the case with rabies, for instance – but other times it might be that, as in the case of Ebola, in a rapidly unfolding outbreak it is impossible to get vaccines to people quickly enough.

Unlike vaccines given to prevent illness, where the immune system is primed to recognise and fight off infection, therapeutic vaccines stimulate the immune system to battle even harder against an existing infection – and the robustness of this response can be the factor that decides whether a patient lives or dies.

Now, therapeutic vaccines are being developed against cancers, viruses such as HIV and the human papillomavirus (HPV) that causes cervical cancer.

Reassuringly, with the Ebola vaccine, having been vaccinated while infected did not affect the ability of people to be treated with monoclonal antibodies – there had been a concern that the antibodies triggered by vaccination and those given as treatment would interfere with each other in some way.

Armand Sprecher, an MSF doctor who has been involved in Ebola outbreak responses for decades but was not involved in this study, told STATnews: “The nice thing is, as they pointed out, that this is not in competition with the therapeutics. So, vaccine plus therapeutic is even better than either alone.”