Vaccination rates among New York City children have dropped by 63 percent, an alarming decline that has health officials worried about a rise in illnesses among the city's youngest residents that could lead to coronavirus infections.

The pandemic has upended many routines, including doctor's visits. Many pediatrician offices have closed, and families have been reluctant to venture outside for fear of exposing themselves and their children to the virus.

On Wednesday, Mayor Bill de Blasio, who announced the statistics during his morning press briefing, urged parents to get their children vaccinated. Unvaccinated children become vulnerable to other illness, including influenza and especially measles. That, in turn, can make them prone to coronavirus.

"We know that anyone with a pre-existing condition is more susceptible to COVID," he said.

"Getting your child vaccinated is a reason to leave your home," he added.

Over the last six weeks, fewer than 150,000 vaccination doses were administered compared to roughly 400,000 doses during the same period last year.

For children 2 and under, the number of vaccinations fell by 42 percent. The drop was most dramatic for children older than 2, for whom vaccinations fell by 91 percent. Most vaccinations are delivered to children between birth and 15 months.

Nationwide, the trend in vaccinations has been similar to that of New York City. Last week, the Centers for Disease Control and Prevention reported that vaccinations administered to 50 percent of U.S. children under a federal program fell substantially after March 13, when a national emergency declaration was issued.

"The identified declines in routine pediatric vaccine ordering and doses administered might indicate that U.S. children and their communities face increased risks for outbreaks of vaccine-preventable diseases," the report said. "Parental concerns about potentially exposing their children to COVID-19 during well child visits might contribute to the declines observed."

The lapse in vaccinations has rekindled fears of another measles outbreak. Last year, New York City saw more than 400 measles cases, mostly in the Brooklyn neighborhoods of Williamsburg and Borough Park where many ultra-Orthodox families had elected not to vaccinate.

On Wednesday, De Blasio sought to allay parents' fears of healthcare settings, saying that hospitals were in a far better position that in March. More than 1,000 city hospital-run facilities offer free vaccinations to children.

"Now is the time to get your child vaccinated," he said. "This is essential work."

Parents or guardians who wish to make an appointment at a city-run healthcare facility should call 844-NYC-4NYC.