While many people in industrialized countries no longer regard measles as a public health threat, the disease still kills more than 1 million people worldwide every year. Many victims are children in developing countries who don’t get vaccinated. Some of the others are people who received a vaccine that was ineffective because it hadn’t been kept refrigerated or because they were less than 9 months old when they received the shot.
Scientists working with rhesus monkeys have now developed an alternative vaccine consisting of copies of just a couple of strands of DNA. It stands up to tropical heat without refrigeration and may confer lasting protection to infants only a few months old, says study coauthor Diane E. Griffin, a virologist at the Johns Hopkins University School of Hygiene and Public Health in Baltimore.
Griffin and her colleagues gave 14 juvenile monkeys two DNA vaccine injections 17 months apart. Of these animals, 11 didn’t come down with measles when exposed to the disease 7 months after the second shot, the researchers report in the July Nature Medicine.
Four unvaccinated monkeys exposed to the measles virus contracted the disease. However, two monkeys that had received the measles vaccine currently in use worldwide proved immune.
The standard vaccine, which was developed in the 1960s, employs a live but disabled version of the virus. This induces immune cells to form antibodies to the virus and retain a memory that triggers a quick protective response during subsequent encounters with measles.
In contrast, the new vaccine delivers a pair of genes that encode two glycoproteins essential to the measles virus but incapable of causing illness themselves. This DNA vaccine produces the proteins within hours of being injected. These compounds, hemagglutinin glycoprotein and fusion glycoprotein, draw the attention of the immune system, which promptly makes antibodies specifically to attack such proteins when it encounters them on a live virus.
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Although the DNA vaccine seems to work, puzzles linger, Griffin says. After being exposed to the measles, the immunized monkeys retained some virus in their bodies. So, they might be largely immune but also still somewhat contagious for the disease, she says.
The researchers also haven’t nailed down the best age at which to administer the DNA vaccine to people. The standard immunization doesn’t “take” early in infanthood because residual antibodies to measles from the mother attack and destroy the weakened virus before the body can muster its own immune response, says James M. Meegan, a virologist at the National Institute of Allergy and Infectious Diseases in Bethesda, Md. These maternal antibodies gradually fade. When children vaccinated as young infants later contact the real virus, they are unprepared, he says.
“Passively acquired maternal antibodies should not affect DNA vaccine uptake and protein synthesis by host cells,” says Ann M. Arvin of Stanford University School of Medicine in the same journal. To ascertain that, Griffin and her colleagues plan to try the DNA vaccine next on infant monkeys.
“We often only get one shot at [immunizing a child] in the developing world, and we’d like that shot to be very early in life,” Meegan says. Even so, he says, measles vaccination may evolve into a two-step process, with a DNA vaccination in the first months of life and a standard immunization later.