Medical practice

Ebola vaccine shows promise in human trials

“Ebola vaccine trial results promising, says manufacturer,” The Guardian reports. Initial results from a trial involving 20 healthy adults found that the vaccine seems to be safe.

The trial was what is known as a phase one trial, which is designed to test if a drug or intervention is safe, rather than whether it is effective against Ebola.

There were some minor side effects – such as mild pain, fever and generally feeling under the weather – but all symptoms resolved after a few days.

Although the purpose of the study was to assess safety, the researchers also measured the antibody levels that had been produced following the vaccine, which gives an indication that it might be effective in granting immunity against infection.

Antibodies of a level similar to that shown to be effective against Ebola in primate studies was seen in 19 people against the Zaire strain of Ebola, and 15 people against the Sudan strain of Ebola.

The effectiveness of this particular vaccine is now being evaluated in larger clinical trials. Of note, large human trials are ongoing into another newly-developed Ebola vaccine that may be effective just against the Zaire strain of Ebola, which is responsible for the current outbreak.

Hopefully, one or both vaccines will be available by 2015, and are likely to be given first to high-risk groups, such as healthcare workers.

Where did the story come from?

The study was carried out by researchers from National Institutes of Health in Maryland, GlaxoSmithKline Vaccines in Belgium and the University of Naples. It was sponsored by the National Institute of Allergy and Infectious Diseases, Vaccine Research Center, Maryland. Some of the authors have a pending patent related to the vaccine, representing a financial conflict of interest.

The study was published in the peer-reviewed New England Journal of Medicine. It was published on an open access basis, so is free to read online.

The UK media have reported the study accurately and emphasised that the results of further larger trials are required before any vaccination programmes can be initiated.

What kind of research was this?

This was a phase one trial, which is the first type of study that is performed on humans to test the safety of a new drug or vaccine. Phase one trials are generally conducted on a small number of people. In this case, a low dose of the vaccine was used to start with to make sure the tests were as safe as possible for the volunteers.

The effects of the vaccine were then monitored. If phase one trials are successful, then the vaccine will progress to phase two trials, which assess the effectiveness of the vaccine.

The Ebola epidemic in West Africa was declared to be an international public health emergency in August 2014. Since then, efforts to develop a vaccine have been sped up. One of them, the cAd3 Ebola vaccine, has been developed over the past three years with the aim of providing immunity to both the Zaire and Sudan strains of Ebola. It was initially effective in a study of macaque monkeys, but this wore off over the following months. Subsequent tests found that longer-term immunity for up to 10 months was improved by giving a booster dose. The first phase one trial of this drug was planned for the beginning of 2015, but this was brought forward because of the rising Ebola epidemic.

What did the research involve?

The Ebola vaccine was tested on 20 healthy volunteers to assess its safety in humans. This group was composed of nine men and 11 women, with an average age of 37 years.

Eligibility criteria for the study were:

  • 18 to 50 years old
  • availability for 48 weeks after enrolment, so they could be clinically reviewed
  • proof of identity
  • able and willing to complete the informed consent process
  • willing to donate blood to be used in future research
  • good general health without clinically significant medical history
  • a body mass index (BMI) of 40 or less
  • normal blood tests

Women who wished to participate had to have a negative pregnancy test and agree to effective birth control for 21 days prior to the study and 24 weeks after injection of the virus.

Each volunteer was paid approximately $1,700 (£1,074).

The first 10 volunteers received a small dose of the cAd3-EBO vaccine by injection into the shoulder muscle. The next 10 volunteers had a dose 10-fold stronger.

To minimise any risk, only one person was injected per day for the first three people in each group.

All participants were then followed up for four weeks to assess any potential side effects and to monitor the immune response.

What were the basic results?

There were no serious side effects or safety concerns. Mild to moderate reported symptoms included:

  • one person had a severe fever of 39.9C and one person had a mild temperature within eight to 24 hours of the higher injection dose; both resolved within one day
  • the blood tests of three people (one low dose, two high dose) showed that the time it took for their blood to clot had roughly doubled; in addition, four people (one low dose and three high dose) had low white blood cell counts (the cells that fight infection) in the days following the injection
  • 10 people had mild tenderness at the injection site, but no-one had any redness or swelling
  • one person felt moderately unwell after the injection and nine felt mildly unwell

The study’s primary goal was to assess the safety of the vaccine in humans, but tests to determine if the vaccine might be effective were also promising at four weeks:

  • nine low-dose volunteers and 10 high-dose volunteers had antibodies against the Zaire strain
  • seven low-dose volunteers and eight high-dose volunteers had antibodies against the Sudan strain

How did the researchers interpret the results?

The researchers concluded that “no safety concerns were identified” in this small study.


This phase one trial of a potential vaccine against two strains of Ebola (from Zaire and Sudan) did not raise any safety concerns. A few minor to moderate symptoms were reported, but all resolved within the four-week period studied.

Further clinical trials to test the effectiveness of the vaccine are currently underway. They will also monitor side effects in larger study groups and over a longer time period. It will be very interesting to see the results of these trials, as the primate studies showed that immunity wore off within a few months, but could be prolonged with a booster dose. It remains unclear how long any such immunity could last in humans.

It is worth stressing that all volunteers were healthy. Therefore, it is important to assess whether the vaccine is safe in more vulnerable groups, such as the very young and very old, or people with a pre-existing health condition.

The researchers also report that a vaccine that has been developed solely to protect against the Zaire strain of Ebola, which is responsible for the 2014 outbreak, is currently being tested on humans in the UK, US, Mali, Uganda and Switzerland.

We expect to see further developments in this field during the first half of 2015.

Analysis by NHS Choices

NHS Attribution