Tests involving three different kinds of Zika vaccines on rhesus monkeys have produced good immune responses and caused no adverse effects, raising hopes for the development of an effective vaccine for use in humans. None of the vaccinated monkeys developed infection when later challenged with Zika virus. Also, the transfer of antibodies at higher doses from vaccinated monkeys to Zika-naïve mice and monkeys was found to offer “complete protection” when the animals were exposed to Zika virus.

Three vaccine approaches

The results of the study by a team led by Dan H. Barouch from Harvard Medical School, U.S. were published today (August 5, 2016) in the journal Science.

Compared with DNA vaccine, the killed virus vaccine was found to be “more potent” in rhesus monkeys.

Zika virus strains from both Brazil and Puerto Rico were used for developing the vaccines. The study used three vaccine approaches — a purified killed Zika virus, DNA from the Zika virus woven into a plasmid, and inserting Zika genes into adenoviruses which infect cells and trigger immune responses.

In addition, passive protection was also tested by using antibodies from vaccinated monkeys to protect Zika-naïve mice and monkeys. While all the mice that got the antibodies at a higher dose were completely protected, one of the two monkeys that got the antibodies was completely protected.

The vaccine study using purified killed virus vaccine was conducted on 16 rhesus monkeys. Eight monkeys got the purified killed virus vaccine and the remaining eight did not get a vaccine. Within two weeks of immunisation, all vaccinated animals developed Zika-specific binding and neutralising antibodies. After a booster vaccine was administered at four weeks, the antibody levels increased substantially. When these animals were exposed to Zika virus, they showed “complete protection”. The virus was not found in blood, urine and other body fluids including cervicovaginal secretions.

In another experiment, 12 rhesus monkeys were immunised with either a DNA vaccine or an adenovirus vector-based vaccine. While the monkeys developed antibodies when both the vaccines were administered, the adenovirus vector-based vaccine “provoked a broader and a more potent antibody response”. Both the vaccines conferred complete protection when the vaccinated monkeys were exposed to Zika virus four weeks after vaccination.

Compared with DNA vaccine, the killed virus vaccine was found to be “more potent” in rhesus monkeys, the authors say. The adenovirus vector-based vaccine offered complete protection after only one dose of the vaccine.

Passive protection through transfer of antibodies from vaccinated animals to Zika-naïve animals offered protection against Zika virus even at relatively low antibody titers.

Phase I human clinical trials using killed Zika virus vaccine and other candidate vaccines are expected to begin later this year.

Published in The Hindu on August 5, 2016