Three different types of Zika vaccine offer full protection against the virus in monkeys.
Researchers from the US and Brazil injected Rhesus monkeys with vaccines already shown to be effective in mice and found they all elicited enough of an immune response to ward off infection of the Puerto Rico and Brazil Zika virus strains.
The work appears in the journal Science.
On the cusp of the Olympic Games’ opening ceremony in Rio de Janeiro, the Zika virus – which started tearing through Brazil late last year, causing the rare but devastating disorder of microcephaly in babies – has been transmitted in more than 50 countries and territories, according to the Centers for Disease Control and Prevention.
Researchers have been racing to push a vaccine through clinical trials post-haste in a bid to keep it spreading further. Human trials in the US by the National Institutes of Health began in the past few weeks, but a commercially available vaccine is not expected until at least next year.
In the meantime, scientists are working out just how much protection potential vaccines bestow and just how much is needed.
When a person is vaccinated, their immune system recognises ingredients in the vaccine as foreign bodies that need to be dealt with.
Immune cells produce proteins called antibodies that “match” and fight the invader. When the pathogen invades again, the immune system is ready – and can ramp up antibody levels to exterminate the infection before it can take hold.
Some viruses, such as HIV, are stubbornly resistant to vaccinations, while others, such as the flu, are warded off with only a single jab. Where does Zika sit on this spectrum?
To find out, Peter Abbink and Rafael Larocca from Harvard University in Boston and colleagues tested three types of Zika vaccine in Rhesus monkeys to see if they worked as well as how much immune protection they provided.
The first vaccine type used a whole but inactive Zika virus to get antibody production going by the immune system.
The second and third used harmless strands of DNA taken from the Zika virus – one in an enclosure of harmless DNA (called a plasmid) and the other in a type of virus called an adenovirus. In these, the plasmid and adenovirus injected the tiny pieces of Zika DNA into human cells which triggered the immune response.
Abbink, Larocca and their team found all vaccinated Rhesus monkeys, when subsequently infected with Zika, fended off the pathogen. But their non-immunised counterparts did not.
The vaccinations had no adverse side-effects and importantly, the researchers saw that the monkeys only needed low levels of antibodies to successfully deal with the virus. The adenovirus vaccine shielded the monkeys after a single jab.
The National Institutes of Health human clinical trial, using the plasmid vaccine, is seeing how many vaccines are needed and how often to confer immunity to the Zika virus.
“Results from both mouse and non-human primate testing are encouraging and support a decision to move forward with our US government, industry and regulatory partners to advance our [Zika virus] vaccine candidate to human trials,” said Stephen Thomas, an infectious disease army physician and co-author on the study.