Zika vaccine: how close?

The U.S. Centers for Disease Control has taken the nearly unprecedented step of telling pregnant women to stay away from parts of Florida to avoid the Zika virus. Florida officials are mobilizing to kill mosquitoes to stem an outbreak that has infected 16 people in the Miami area.  Both developments have put greater urgency on efforts to develop a vaccine. 

Last week, the U.S. National Institute of Allergy and Infectious Diseases (NIAID) announced the launch of a phase 1 clinical trial of a DNA vaccine against the Zika virus. Eighty healthy volunteers recruited from Maryland and Georgia will receive at least one injection of the vaccine.  The first volunteer was given the test vaccine last Tuesday (Aug 2). Some of the participants will receive a second shot eight or 12 weeks later, while others will receive a second and a third shot four and eight weeks or four and 20 weeks following the first shot.  Test subjects will be monitored for two years to determine if the vaccine is safe, and if it causes the immune system to make antibodies capable of fighting Zika.  Another DNA vaccine developed by Inovio Pharmaceuticals has also just begun human testing. The latter vaccine has a Canadian connection; the University of Laval is one of three testing sites.

Last week, a team of scientists from the U.S. and the University of Sao Paulo in Brazil tested two potential vaccines in rhesus monkeys.  One of the vaccines is made from a purified inactivated Zika virus; the other is a DNA like the one that's being tested in humans. Both vaccines protected the monkeys against the Brazilian strain of Zika, but the DNA vaccine had a stronger effect. The results were published last week in the journal Science.  The results come on the heels of another study published late in June in the journal Nature showing either type of vaccine gave mice nearly total immunity against the Zika virus.

Human vaccines are usually made from live viruses that are attenuated or modified so that they trigger your immune system to make antibodies without giving you a dose of the disease itself.  The vaccine contains antigens that trigger your immune system to make antibodies to attack the antigens and hopefully attack the virus.  The method – while time honoured – is somewhat crude and prone to hit and miss.  A DNA vaccine is tiny bit of the Zika virus's genetic code.  Instead of injecting the body with a antigens from the virus, a DNA vaccine gets the body to use the foreign bit of DNA to make an antigen – when in turn triggers the immune system to make antibodies that attack the Zika virus.  DNA vaccines are supposedly easier and quicker to manufacture, and can be targeted to fight an infection with pinpoint accuracy.

DNA vaccines only work if the DNA triggers the production of a substance that in turn gets your immune system to make antibodies that attack the virus – in this case, Zika. Some researchers are concerned that a DNA vaccine might trigger the immune system to make antibodies that attack DNA itself. Others say a DNA vaccine – which is a piece of genetic material – might have some unexpected impact on other genes. So far animal and human studies have shown DNA vaccines to be safe and well tolerated, but experts know very little about the long-term impact of DNA vaccines in humans. 

Another 'risk' has to do with funding. Last week, NIAID Director Dr. Anthony Fauci called on the U.S. Congress to approve emergency funding for the vaccine.  In a conference call with reporters, he said clinical trials of the vaccine may have to be delayed.

Meanwhile, Florida officials have started a massive campaign to kill the Aedes aegypti mosquitoes that carry Zika. They've dispatched personnel to search for and remove pools of standing water where mosquitoes breed. The U.S. Food and Drug Administration has approved a field trial of genetically engineered mosquitoes that would limit the population of Aedes aegypti mosquitoes. The male OX513A mosquitoes would mate with wild females, but their offspring would die before reaching adulthood.  The FDA says the genetically modified mosquitoes pose no significant environmental risk.  The trial still has to clear local and state regulatory hurdles.

Ultimately, the best prevention is a vaccine. If the phase 1 vaccine results are encouraging, the next step is a phase 2 clinical trial to take place in countries with ongoing Zika outbreaks.  That study could begin early next year.

When a vaccine is available, I'll be rolling up my sleeve.  Will you?

Dr. Brian Goldman is an ER physician in Toronto, and host of White Coat, Black Art, which returns next month with new episodes.

For the latest figures on the outbreak of Zika virus in Florida, click here.