Your questions, answered "Are there any other vaccines that require four doses? It seems to me that is an extreme number and perhaps the initial vaccine was too rushed." —Susan in Georgia Yes, there are a number of vaccination series that require multiple doses of three, four or more — including some that call for regular boosters throughout our lifetimes. Many of us may not think about them because we receive them during childhood to prime our immune systems to fight diseases we may encounter as children or later on as adults. But health experts say there is precedent, both for children and adults, to get repeated doses of vaccines. Ruth Karron, a professor and director of the Johns Hopkins Vaccine Initiative at the Bloomberg School of Public Health, said these vaccines are often broken down into a primary series and a booster dose or doses. The diphtheria, tetanus and acellular pertussis vaccine (DTaP), Haemophilus influenzae type b vaccine (Hib), pneumococcal conjugate vaccine and inactivated polio vaccine, for instance, require three initial doses and then a booster shot — or a fourth dose, according to the child and adolescent immunization schedule from the Centers for Disease Control and Prevention. DTaP calls for a fifth dose later in childhood. In adulthood, a fourth dose of the hepatitis B vaccine is recommended for people who are at higher risk. Tetanus boosters are administered every 10 years. And, of course, many of us receive the annual influenza vaccine dozens of times throughout our lives. Granted, the flu shot is a bit different in that each year it is tailored to provide protective immunity against a new variant — which is one reason we get it. The point is, needing a series of doses does not mean the vaccine research was rushed. Messenger RNA technology had been studied, developed and refined for years before it was used in the coronavirus vaccines, and experts said no corners were cut in bringing them to market. Perhaps the bigger question is whether the coronavirus vaccine will ultimately consist of a primary series and booster, or become an annual vaccine. Experts said it is too early to know. The vaccine has been effective at preventing severe disease and death — even against the highly contagious omicron variant — and although antibodies wane, vaccine-induced immunity could last years, said William Moss, executive director of Hopkins' International Vaccine Access Center. Most people "may not need additional doses or frequent additional doses," he said. However, if it turns out SARS-CoV-2 — the technical name for this coronavirus — develops into a seasonal respiratory virus such as influenza, Moss said annual boosters may be needed to protect against potential new variants. And if the virus evolves into a version capable of escaping all our prior immunity to severe disease and death, then we may need additional doses or a new version of the vaccine, he said. Karron said she understands some people may be frustrated by the lack of immediately available answers. "It is true we are learning as we go. But we've also been able to apply these lessons in real time very effectively," she said. | 
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