Patients don’t quite share that rosy view. The shortfalls are particularly irksome to those who have received the first shot and been unable to get the second one.

Ralph Stern, 81, in Orange, Conn., said he and his wife had their first injections in April but have not been able to track down a second dose, even though they have tried the recommended vaccine-finder and called drugstores and supermarkets that are supposed to have the vaccine.

“They give you a pharmacy and you call the pharmacy and they don’t have it,” Mr. Stern said.

An acquaintance of Mr. Stern’s lost six months of work because of shingles, and that motivated him and his wife to be vaccinated, he said. It worries him that they may not be able to get the second shot inside the recommended six-month time frame.

“I want to be in the window where you had a record of success,” he said. “I want what they recommended, and then they can’t do it.”

Dr. Kathleen L. Dooling, a shingles expert at the Centers for Disease Control and Prevention, said that in studies, the vast majority of patients received the second injection two to six months after the first, so the best evidence is based on that timing. People who miss the six-month mark should just get the second shot as soon as possible, she said. But there is no reason to start over again: a total of two shots is still enough.

She said small studies looked at people who received the second shot six to 12 months after the first, and while the overall immune response remained strong, “there was some individual variation.”

She said the best strategy was for people receiving the first shot to make an appointment two months later for the second one, to try to lock in that dose. She said the disease centers had encouraged providers to give priority to patients seeking their second shot.