The study used data from the Michigan Care Improvement Registry, which tracks immunizations within the state. It compared vaccination rates for children 5 months or younger on a typical day in May from 2016 to 2019, with the same day this year. It showed that before the pandemic, roughly two-thirds of children in that age group were up to date with their vaccinations; this year, the rate fell to 49.7 percent.

The study also showed that Michigan children on Medicaid were even less likely to be current on their immunizations. The largest disparity was seen among those 7 months or younger. The researchers found that only 34.6 percent children on Medicaid were up to date, compared with 55 percent of children in Michigan who were not on Medicaid.

Dr. Shen said the falling rates could jeopardize the herd immunity that communities have built up against a disease like measles. Public health officials estimate that a community vaccination rate from 93 percent to 95 percent is necessary to prevent a widespread outbreak of measles.