Ebola Vaccine: Is It Safe?

The outbreak of the Ebola virus in Africa (2013-2016) caused great concern among medical personnel. At that time, there was no vaccine available against this frequently fatal disease, and there was some concern that the virus may be spread to many different countries with populations highly susceptible to this disease. Fortunately, the disease outbreak (approximately 28,000 infected with 11,000 deaths) was eventually contained without causing a pandemic. However, almost immediately, scientists began to explore ways to develop an effective vaccine to protect humans from this virus. In the past, many vaccines have been developed that utilize a modified version of the causative organism to allow humans to develop an immune response that is protective against the disease; but with many organisms, this has been unsuccessful and sometimes even dangerous for the recipient of an experimental vaccine. It is not surprising that the question is asked about any new experimental Ebola vaccine...Is it safe?

There are a number vaccines that are being tested experimentally in an effort to produce a safe and effective vaccine for use in humans. There are seven vaccines that utilize a "vectored glycoprotein" antigen from Ebola viruses that has been placed into a virus that will introduce it to the immune system without the ability to cause Ebola, since there are no Ebola replication genes in the glycoprotein, which is a protein made by the virus. There is one vaccine that uses a replication-defective Ebola virus (EBOV∆VP30) further treated with hydrogen peroxide to make sure it is unable to replicate. All of the vaccine researchers suggest their vaccines are safe to use. However, most vaccines are still going through animal or clinical trials to prove safety and efficacy. In December 2016, the first report of a large vaccine trial that showed safety and effective immunity against the Zaire strain of Ebola in humans during an outbreak of this disease, rVSV-ZEBOV vaccine, was published.

The new experimental Ebola vaccine is termed rVSV-ZEBOV and is manufactured by Merck, Sharpe & Dohme. Instead of using Ebola virus that has been weakened or attenuated, the scientists developed a virus that was able to replicate, cause no disease in humans, and was genetically modified to express a surface glycoprotein produced by the Zaire Ebola virus strain. Clinical testing of this recombinant virus vaccine was begun in 2015. The trial involved 11,841 people; 5,837 of them received the vaccine immediately (day 0), and the remainder did not receive the vaccine until 21 days after exposure to Ebola. The study was done in Guinea, West Africa, where an outbreak of Ebola was occurring. People involved in the trial were individuals who had close contact with other people known to have laboratory diagnosed Ebola and thus were likely exposed to the disease. The method was modeled after the technique used to eliminate smallpox. Such individuals were randomly assigned to either immediate vaccination (day 0) or delayed vaccination (21 days later). The results of the experimental trial were that the immediate vaccine offered substantial protection against the disease, with no cases occurring among vaccinated individuals 10 or more days after immediate vaccination while 23 individuals who obtained the delayed vaccination contracted Ebola. Analysis of Ebola cases occurring 10 days after receiving the vaccine was done to account for the incubation period the Ebola virus.

To assess safety of the vaccine, individuals who were vaccinated were observed for 30 minutes after vaccination in at-home visits on days three, 14, 21, 42, 63, and 84. Headache, fatigue, and muscle pain were common mild symptoms that occurred soon after vaccination but were treated with acetaminophen (Tylenol) or nonsteroidal anti-inflammatory drugs (NSAIDs). Most individuals recovered within a few days without any long-term problems. Two serious adverse events were noted and were judged to be related to the vaccination; one individual developed a fever, while another experienced anaphylaxis. Another patient who received the vaccine developed a flu-like illness. All three patients recovered completely.

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All of the vaccine researchers involved in the various types of vaccines claim they are safe because the vaccines do not contain either Ebola viruses or Ebola virus genes that allow the virus to replicate and cause disease. However, until each vaccine has been tried appropriately in humans, most doctors and vaccine researchers are withholding judgment until they see the data on effectiveness, side effects, and overall safety. Safety (and risk of side effects) is relative to the individual. If side effects and overall safety of the vaccine produces small risks relative to catching the disease that may be lethal, then most vaccine researchers and doctors would consider the vaccine relatively safe. For many doctors and researchers, the data produced by vaccine studies done with rVSV-ZEBOV vaccine has met safety criteria and is considered a relatively safe and effective anti- Ebola vaccine. However, some researchers are unsure if this vaccine will protect humans from all strains of Ebola viruses because of some differences in the proteins on their surfaces.