SEATTLE — Researchers at the Walter Reed Army Institute of Research and Harvard Medical School reported the success of three experimental vaccines to prevent Zika in monkeys. The vaccines were already proven to be effective in mice, but given their genetic similarity to humans, monkeys are far more telling of whether a Zika vaccine will be effective on humans.
It’s been a year since Zika grabbed international attention for its association with microcephaly, a disease that causes babies to be born with underdeveloped brains and can ultimately cause severe neurological disorders. Since April of 2014, Brazil reports almost 5,000 cases of babies born with Zika-related birth defects. Global health officials have continued to study the disease as it spread throughout South America, Central America, North America, Asia and Europe.
We know that Zika is transmitted to humans through mosquitoes and is characterized by relatively mild symptoms, such as a rash, fever, red eyes and joint pain. After the outbreak last summer, newer research revealed that Zika can be sexually transmitted.
Viral strands of RNA can stay in men’s sexual reproductive system for up to two months after exposure, even if they don’t experience symptoms. We also know that basic precautions, such as protective clothing and insect repellent, can protect people in endemic regions from being bitten by possibly-infectious mosquitoes.
Despite the alarming prevalence of Zika in Brazil, the Rio Olympic Games continued as scheduled. The World Health Organization (WHO) rejected a request from over 100 scientists and medical professionals to postpone the games, stating, “Brazil is one of almost 60 countries and territories which to date report continuing transmission of Zika by mosquitoes… People continue to travel between these countries and territories for a variety of reasons. The best way to reduce risk of disease is to follow public health travel advice.”
For the most part, athletes and travelers have taken that advice. The tone of the Zika discussion is far less fearful than it seemed in the summer of 2015. Recent efforts in research and development have even produced three experimental versions of a Zika vaccine to guard against the disease and its dangerous effect on unborn babies.
The Walter Reed Army Institute and Harvard researchers injected eight control-group monkeys with a placebo and another eight with a vaccine that contained an inactive version of the virus. After two weeks, the vaccinated monkeys produced Zika-specific antibodies that successfully protected them from the virus. The team also developed two other vaccines: a DNA vaccine and a recombinant adenovirus vector vaccine. All three proved safe and effective and are licensed by several major regulatory agencies.
The are currently three effective vaccines, so which one is best for humans? Ultimately, the Zika vaccine containing inactive Zika RNA is the best candidate. DNA vaccines contain genetically modified DNA that triggers an appropriate response by the human immune system. They are extremely long lasting and don’t require booster shots.
The problem is that a monkey would need to be administered 5 milligrams of the DNA vaccine (compared to 5 micrograms of the inactivated virus vaccine). Additionally, a vaccine that is concentrated would be like injecting syrup into a human vein- which is painful, difficult, and sticky.
Adenovirus vaccines employ the same principle as DNA vaccines. They insert themselves into human DNA, which allows the immune system to generate antibodies that guard against viruses. However, just like the DNA vaccine, the adenovirus vaccine requires a much larger quantity per dose to be effective in humans.
A batch of the inactivated Zika vaccine is ready for clinical trials. In July, the U.S. Army even agreed to transfer its vaccine technology to Sanofi SA, a manufacturer in Paris. The new partnership will aid in developing the 100 million doses necessary to treat Zika in endemic regions.
It could be three or four years before a Zika vaccine arrives at primary care providers. Nonetheless, the research team at Harvard has taken an incredible leap forward in the global effort to control Zika.
– Jess Levitan