The earliest vaccines were not patented, in part because the law at the time held that natural products could not be so protected. And vaccines like polio were developed through a large infusion of government and foundation funds, not by a company. Even when commercialized by the 1960s, vaccines were made by small specialty manufacturers, instead of big pharmaceutical firms, since producing them involved particular challenges: using live organisms, some of them dangerous. Indeed, huge liability payouts and aggressive mergers had, by the 1990s, meant that more than half of the country’s vaccine makers had closed down. With low retail prices, no one regarded vaccine making as a lucrative business.

When he started his pediatric practice in 1982 in San Antonio, Dr. Michael Ozer remembers, he charged $22 for a 2-month well-child checkup, with $8 added on for the polio vaccine and another $8 for the vaccine against diphtheria, pertussis and tetanus. “And I’m sure we were making money on it,” he said.

But one by one, various barriers eroded: Drug manufacturers discovered new ways to protect their products, like patenting the manufacturing process. The number of vaccine patent applications rose tenfold in the 1990s to more than 10,000. In 1988, the federal government set up the Vaccine Injury Compensation Program, effectively shielding manufacturers and doctors.

And the acceptable list price for drugs was rising. Vaccines, which families often used to pay for out of pocket, are now typically covered by insurance, and patients often do not notice the prices.

There are, of course, some good reasons vaccines like Prevnar are more expensive than previous offerings. Vaccine trials, which once included thousands of volunteers, must now include tens, if not hundreds of thousands of people, as fears about side effects like autism have grown, even though many studies have concluded that such worries are unfounded. Some of the newer vaccines are complicated to manufacture.

Image Oskar Lozano, 4 months old, gets a checkup by Dr. Michael Ozer before getting his vaccines in San Antonio. Credit... Ben Sklar for The New York Times

Prevnar, for example, involves attaching a piece of a dangerous bacterium’s outer layer to a protein that renders it better able to provoke a protective immune reaction in babies. And because it covers 13 strains of the disease, it is in some ways 13 vaccines in one.