The monoclonal antibodies and maternal vaccination both protect babies through passive immunization, which means the infants are given antibodies. Solving that mystery took many years and led to the successful development of today’s monoclonal antibodies and RSV vaccines for pregnant and older adults. But the chilling effect on pediatric RSV vaccine development following the 1960s disaster has never fully dissipated. “With the live, attenuated vaccines, the immune response is more like the immune response you get to a natural infection,” Cunningham says. And that’s further than almost any other RSV vaccine has gotten since those fateful clinical trials six decades ago.