Second Dose Of COVID-19 Vaccine: How Long Can I Wait?

Toni McAllister
·4 min read

RIVERSIDE COUNTY, CA — With concerns about a shortage of COVID-19 vaccines across Riverside County and the rest of the nation, a local health official sought to ease concerns about getting the second shot in the series at the exact prescribed time. But there are conflicting reports of just how long one can wait between the first and second doses.

What are the recommendations?

The Pfizer-BioNTech and the Moderna COVID-19 vaccines are being distributed in Riverside County, and both are reportedly around 95 percent effective. Two doses — an initial inoculation and a second dose — are needed to provide full protection against the coronavirus. The two different vaccines cannot be interchanged.

Pfizer-BioNTech is recommended for people 16 and older, according to the Centers for Disease Control and Prevention. Pfizer advised that its two shots be given 21 days apart. If the initial inoculation was a Pfizer-BioNTech vaccine, the second shot must be the same.

The Moderna COVID-19 vaccine is authorized and recommended for persons 18 and older, according to the CDC. Moderna advised that its two shots be given 28 days apart. If the initial inoculation was a Moderna COVID-19 vaccine, then the second shot must be the same.

Riverside County health official weighs in

Riverside County has received a total of 168,575 vaccine doses from both manufacturers, with more than 75 percent going to county partners, according to Riverside University Health System-Public Health.

The system's Dr. Geoffrey Leung said in a weekly livestream update Thursday that there is no danger if someone receives a first dose of either vaccine and doesn't get the second shot right on time.

Amid concerns over vaccine shortages, "there may be some uncertainty on when you can get that [second] vaccine scheduled," Leung said. "You can get that second dose anytime at three weeks or four weeks or afterward. It's safe at even two, three, four months out."

Leung did not discuss a vaccine's efficacy rate if a second dose is delayed.

CDC, FDA statements

The CDC provided an update Thursday to its vaccine recommendations. The CDC said people should not receive a second dose earlier than recommended (three weeks for Pfizer-BioNTech or one month for Moderna).

The CDC also emphasized that the second dose should be administered as close to the recommended date as possible but acknowledged that's not always feasible. "The second dose of Pfizer-BioNTech and Moderna COVID-19 vaccines may be scheduled for administration up to six weeks (42 days) after the first dose. There are currently limited data on efficacy of mRNA COVID-19 vaccines administered beyond this window," the CDC reported.

If a second dose is administered beyond these intervals, "there is no need to restart the series," the CDC said.

The Food and Drug Administration addressed the second-dose timetable on Jan. 4. "In the phase 3 trials, 98 percent of participants in the Pfizer-BioNTech trial and 92 percent of participants in the Moderna trial received two doses of the vaccine at either a three- or four-week interval, respectively," the agency said.

"Those participants who did not receive two vaccine doses at either a three-or four-week interval were generally only followed for a short period of time, such that we cannot conclude anything definitive about the depth or duration of protection after a single dose of vaccine from the single-dose percentages reported by the companies," the FDA said.

Pfizer's official statement

Pfizer provided a statement to Patch Monday about second-dose intervals. Not surprisingly, the company did not veer from its vaccine label indications.

"Recommendations on alternative dosing regimens reside with health authorities and may include recommendations due to public health principles," Pfizer said in a statement. "As a biopharmaceutical company working in a highly regulated industry, our position is supported by the label and indication agreed upon with regulators and informed by data from our Phase 3 study.

"Pfizer and BioNTech’s Phase 3 study for the COVID-19 vaccine was designed to evaluate the vaccine’s safety and efficacy following a 2-dose schedule, separated by 21 days. The safety and efficacy of the vaccine has not been evaluated on different dosing schedules as the majority of trial participants received the second dose within the window specified in the study design," the company added.

"Data from the Phase 3 study demonstrated that, although partial protection from the vaccine appears to begin as early as 12 days after the first dose, two doses of the vaccine are required to provide the maximum protection against the disease, a vaccine efficacy of 95 percent," the company said.

Moderna's official statement

Moderna did not immediately respond to Patch's request for comment. The company has publicly said that the recommended "two-dose regime of the Moderna COVID-19 Vaccine given 28 days apart was well-tolerated and demonstrated vaccine efficacy of 94.1 percent against COVID-19."

This article originally appeared on the Murrieta Patch