The National Institutes of Health has begun testing an experimental Zika vaccine in at least 80 healthy adults, moving a candidate designed earlier this year into human studies as local mosquito transmission appears in Florida. The early-stage trial will examine safety and whether the DNA-based vaccine produces an immune response; it cannot yet determine whether the product prevents infection.

Volunteers ages 18 to 35 are enrolling at three sites, including the NIH Clinical Center in Bethesda, Maryland. They will be divided into four groups of 20 and receive two or three injections on different schedules. The NIH announcement on Wednesday says researchers expect preliminary results by January.

A vaccine built from genetic instructions

The candidate contains a circular piece of laboratory-made DNA called a plasmid. Scientists at the National Institute of Allergy and Infectious Diseases’ Vaccine Research Center inserted genes encoding Zika proteins. After injection into muscle, human cells are expected to produce virus-like particles that cannot cause infection but can teach the immune system to recognize the virus.

The approach avoids using live Zika virus and allows relatively rapid design and manufacturing. It draws on work with a West Nile virus DNA vaccine tested previously by the same center. No DNA vaccine is currently licensed for human use in the United States, so the platform’s speed must be balanced against limited commercial experience.

The registered Phase 1 protocol describes the primary goals as safety and tolerability, with immune response as a secondary measure. Participants will remain under observation after each injection, record symptoms and temperature at home, and return for blood tests. Researchers will monitor them for two years.

Safety first, efficacy later

Phase 1 trials are designed to identify common immediate reactions and evidence of harmful effects in a small group. Investigators will measure binding and neutralizing antibodies and other immune responses, but an antibody result alone does not prove protection. A larger trial in an area where Zika is circulating would be needed to compare infection rates.

The Washington Post’s account of the launch reported that NIAID hopes to begin a larger study in Zika-affected countries early next year if the candidate is safe and immunogenic. That schedule is unusually fast for vaccine development but reflects the outbreak’s urgency and the ability to build on related flavivirus research.

The NIH study is not the first Zika vaccine to enter a human trial. Inovio Pharmaceuticals and GeneOne Life Science announced last week that they had dosed the first participant in a 40-person study of another DNA candidate. Inovio’s July 26 statement says that open-label, dose-ranging trial will assess safety, tolerability and immune responses using an electrical-pulse delivery device.

Animal evidence supports cautious optimism

Multiple approaches have protected mice against experimental Zika challenge, including DNA and purified inactivated-virus vaccines. Walter Reed Army Institute of Research scientists reported in June that a single dose protected mice and generated neutralizing antibodies. An Army account of the program described subsequent testing in monkeys and plans for human studies.

Results published this week add evidence in nonhuman primates. Three vaccine platforms—purified inactivated virus, DNA and a viral vector—protected rhesus monkeys from detectable infection after challenge. CIDRAP’s August 5 summary emphasized that primate protection is an important step because the model more closely resembles human infection than mouse experiments.

Animal success does not guarantee human efficacy or safety. Zika belongs to the flavivirus family with dengue, yellow fever, West Nile and Japanese encephalitis. Researchers must determine whether vaccine-induced antibodies remain protective over time and whether immune responses interact adversely with related viruses. They must also study women of childbearing age and eventually pregnancy, the population for whom prevention is most consequential.

Florida increases urgency but not speed limits

The trial began days after officials identified the first local mosquito-borne transmission in the continental United States. Four Florida patients were initially linked to a small area north of downtown Miami; investigators have since identified additional infections and advised pregnant women to avoid the affected zone. Puerto Rico has a much larger ongoing outbreak.

Zika infection is usually mild or asymptomatic, but infection during pregnancy can cause microcephaly and other severe brain defects. The virus can also spread through sex and has been associated with Guillain-Barré syndrome. There is no licensed vaccine or specific antiviral treatment.

Reuters reported Wednesday that the Florida cases and expanding epidemic have intensified pressure to move quickly. Even so, researchers cannot skip the staged process: a product given to healthy people must demonstrate a strong safety profile, and efficacy requires enough exposure in a trial population to produce a meaningful comparison.

Funding threatens the next phase

NIAID Director Dr. Anthony Fauci said the institute can complete the current study with redirected funds but may not be able to begin the larger Phase 2 trial on schedule. Congress adjourned without approving the administration’s $1.9 billion emergency request or an alternative package. Federal agencies have shifted money from Ebola preparedness and other programs to maintain current Zika work.

A second Reuters report said NIH expects to exhaust $47 million in repurposed funds by the end of August, while other components of the response face similar deadlines. ABC News quoted Fauci warning that the financing gap could delay Phase 2, when the candidate would be tested in many more people in affected locations.

Timing matters because vaccine trials need active transmission to measure protection. If the epidemic subsides in one region, investigators may have to move sites or enroll more participants, raising cost and complexity. Manufacturing enough doses under clinical standards must also begin before efficacy is known if the next phase is to start promptly.

Time’s contemporaneous description of the study outlines the four dosing groups and planned immune testing. The design is a beginning rather than a breakthrough: 80 volunteers can show whether a candidate deserves to advance, not whether it can stop Zika. The significance lies in translating months of laboratory work into the first human data for the government’s leading DNA approach.

If the vaccine proves safe and produces strong neutralizing responses, NIH can move toward an efficacy trial in 2017. If it does not, the parallel programs using inactivated virus, other DNA constructs and viral vectors provide alternatives. The scientific portfolio is broadening quickly; whether the public program can maintain that pace now depends as much on appropriations as on immunology.