Bundibugyo Ebola explained: why the strain now in Nigeria has no vaccine

What is Bundibugyo virus?

It is one of the four ebolaviruses known to cause Ebola disease in humans.

Ebola is not a single virus. Scientists group the viruses that cause it into species, of which the best known is Zaire ebolavirus, behind the West African epidemic of 2014 to 2016 and most outbreaks in Congo. Bundibugyo is a rarer relative. It was first identified in 2007, after an outbreak in Bundibugyo District in western Uganda, near the Congolese border. It surfaced again in 2012 in Congo’s Province Orientale, where it infected 59 people and killed 34.

In those earlier outbreaks, between 30% and 50% of the people infected died.

Why is there no vaccine?

Because the vaccines that exist were built for Zaire, and Bundibugyo is different enough that protection cannot be assumed.

The two licensed Ebola vaccines, including Merck’s single-dose rVSV-ZEBOV, target the Zaire species. Bundibugyo is a close cousin, but it has different genetic material and different surface proteins, the parts of the virus that the immune system learns to recognise. Studies in monkeys suggest some cross-protection, but scientists say that cannot be relied on to protect people in an outbreak.

When the current outbreak began, there were two candidate vaccines for Bundibugyo, one based on the same rVSV platform and one on the ChAdOx platform used for the Oxford Covid-19 vaccine. In May neither had been tested in people, and producing trial doses was expected to take months. The World Health Organization advised that candidate vaccines and treatments be used only within clinical trials, so that the outbreak would show whether they work.

How does it spread, and what are the symptoms?

Through close contact with the blood or body fluids of a sick person. It can also spread through contaminated surfaces and materials. Fruit bats are thought to be the virus’s natural host. People are infectious only once they have symptoms, which appear between two and 21 days after infection.

The early signs are fever, tiredness, muscle pain, headache and a sore throat, which is why the disease is easily mistaken for malaria or other fevers. Severe cases bring vomiting and diarrhoea, organ failure and bleeding. There is a further problem: rapid field tests designed for the Zaire and Sudan viruses can miss Bundibugyo.

How big is this outbreak?

The biggest Bundibugyo outbreak ever, and already the most widespread Ebola outbreak of any kind.

It began in the Democratic Republic of Congo in May, where more than 9,500 cases have been recorded, about half of them fatal. It has since crossed into Uganda and Kenya. Kenya’s nine cases all belong to one family cluster linked to a man who had lived in Congo and died in Nairobi on 5 October. Jean Kaseya, director-general of the Africa Centres for Disease Control and Prevention, said the outbreak “has already become the most geographically extensive in history”.

Why does the Nigerian case matter?

Because it shows the virus travelling by air, far from the outbreak’s centre.

The patient flew into Nigeria from Uganda and was treated at a private hospital for several days before Ebola was confirmed. Those are the days when a patient can infect family members and health workers who do not know what they are dealing with. Nigeria is tracing 44 close contacts and has put every health facility on alert. It has done this before: in 2014 it stopped a Zaire Ebola chain that arrived by air in Lagos. This time health workers must contain it without a vaccine to ring around the contacts, relying on the older tools of isolation, tracing and protective equipment.