(NEXSTAR) — Only a small percentage of people receiving the two-dose COVID-19 vaccines have been shown to miss the second shot, according to data from the Centers for Disease Control and Prevention.

Nearly 9 in 10 people received the second COVID-19 vaccine within the recommended interval, according to an analysis of people vaccinated between Dec. 14 — when the COVID-19 vaccine distribution began — and Feb. 14.

Right now, Pfizer and Moderna, two of the three COVID-19 vaccines authorized for emergency use in the U.S., require two shots each to reach full vaccination. The third vaccine — Johnson & Johnson — requires only one shot.

The recommended interval between doses is 21 days for Pfizer and 28 days for Moderna. However, up to 42 days between doses is allowed when a delay cannot be avoided.

The data showed that 3.4% of the more than 12 million people in the U.S. who received the first of two shots failed to get the second one within the 42-day maximum interval between injections.

Among people who received the first dose, 88% received the second shot to complete the series, while 8.6% had not received the second dose but were within the allowable interval to do so. Among all two-dose recipients, 95.6% received the second dose within the recommended interval between doses.

Missed doses and second doses given outside the recommended interval time were infrequent but varied by jurisdiction and demographic groups, the CDC said.

“Public health officials and providers should work to better understand the reasons for lack of completion of the COVID-19 vaccination series and early and delayed intervals,” the CDC said. “Continued monitoring of series completion status across jurisdictions and by demographic characteristics is important to ensure equity in vaccine administration and vaccination coverage, especially as vaccination efforts expand to additional population groups.”

While doctors have said vaccines can be effective in a single dose, health officials continue to urge people to receive the second dose to maximize the efficacy of the vaccine.

Tests showed the Pfizer and Moderna vaccines were 95% effective in protecting against symptomatic COVID-19 after two doses. J&J has said its vaccine was 72% effective against moderate to severe COVID-19 and 85% against the most serious symptoms.

Studies have shown that a single shot of the Pfizer vaccine is 52% effective at preventing COVID-19 infection. The Moderna vaccine is thought to be 80.2% effective after a single dose, according to a document the vaccine manufacturer submitted to the FDA.

Since COVID-19 vaccine distribution began in the U.S. on Dec. 14, more than 124 million doses have been given, with over 44 million people — or 13.3% of the population — becoming fully vaccinated, the NPR vaccine tracker showed Monday.