Coronavirus: What happens if the second dose of your COVID-19 vaccine is delayed?

Several COVID-19 vaccination providers have been forced to cancel appointments in the last week because of delayed delivery of doses of the vaccine.

While most of those who had their appointment canceled were set to get their first dose of the vaccine, the cancellations are also concerning for those who are coming up on the scheduled date for their second dose.

What happens if you can’t get the second dose of the vaccine within the manufacturer’s prescribed time frame due to a delay in delivery of the vaccine?

Here is what we know about what happens if the second dose is delayed

What are the recommendations for vaccine dosing?

The COVID-19 vaccines being used now in the United States are the Pfizer/BioNTech vaccine, BNT162b2, and Moderna’s vaccine, mRNA-1273.

The vaccines are mRNA vaccines, or vaccines that instruct your body’s cells to produce proteins that mimic those found in viruses. Those proteins trigger immune cells that lead to the production of antibodies that will fight the virus should you be infected.

Both vaccines require two doses to reach an approximate efficacy rate of around 95% each.

The Pfizer vaccine requires two doses 21 days apart. The Moderna vaccine requires two doses 28 days apart.

How effective is just one dose?

The first dose of a vaccine is designed to train your body to recognize the virus and ramp up the immune system, the body’s defense against disease.

The second shot is designed to strengthen that immune response.

What we know is that the vaccines are very effective against the novel coronavirus if they are given at the proper intervals. What we don’t know is how effective they are if the doses don’t come at the proper intervals.

We don’t know how the effectiveness of the vaccines is affected if the doses are not given at the proper intervals because during clinical trials the final effectiveness of the vaccines was measured after having two doses spaced apart by either 21 or 28 days.

We do know that according to information provided to the Food and Drug Administration by Pfizer, their vaccine is 52.4% effective after the first dose. The second dose boosts that effectiveness to 94.8%.

The Moderna vaccine has even higher numbers. It has been shown to be 80.2% effective a little more than a week after one dose, according to information provided to the FDA. After two doses, it is 94.6% effective.

Moderna claims their results show even higher effectiveness after one dose, 86.4%, in those over age 65.

However, the FDA noted that the evidence about the lasting efficacy of a single dose is limited and more testing would be required to determine how long a single dose of a vaccine would offer protection.

If the numbers are so high with the first dose, why worry about a second dose?

The effectiveness of the drugs is based on clinical trials where participants received both doses of the vaccine at the intervals set by the drug makers.

Clinical trial participants who did not get a second dose were followed only for a short period of time, so researchers have no way of knowing if one dose of the vaccine provides long-lasting immunity from the virus.

“In the phase 3 trials, 98% of participants in the Pfizer-BioNTech trial and 92% of participants in the Moderna trial received two doses of the vaccine at either a three- or four-week interval, respectively,” according to a statement from the FDA.

“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.”

What happens if you cannot get the second dose of the vaccine within the recommended time period?

First, don’t panic. If you have had the first shot, you are in a better position than if you have not had it.

Moncef Slaoui, an immunologist and leader of Operation Warp Speed, the federal government’s vaccine development effort, said in a December news conference that when it comes to the timing on the second dose, “There is the regulatory answer and there is the scientific observation.”

Slaoui said that while the immune system generally responds better when there is more time between vaccinations, but the fact that the world is facing a pandemic should cause us to act differently.

“If there is significant transmission of disease, as is the case here, we should absolutely get the second dose exactly as has been studied,” he said.

While the Centers for Disease Control and Prevention recommends the second dose of the vaccine be administered on schedule, the agency says that if that can’t happen then the second dose can be given up to 42 days after the first. The CDC reports that there is not enough data to determine if the efficacy rate remains at the original level after 42 days.

“The second dose should be administered as close to the recommended interval as possible,” the CDC said in a statement. “However, if it is not feasible to adhere to the recommended interval, the second dose of Pfizer-BioNTech and Moderna COVID-19 vaccines may be scheduled for administration up to 6 weeks (42 days) after the first dose.

“There are currently limited data on efficacy of mRNA COVID-19 vaccines administered beyond this window. If the second dose is administered beyond these intervals, there is no need to restart the series.”

According to the British Columbia Medical Journal, “Based on unpublished data available to the National Advisory Committee on Immunization through Health Canada for both the Pfizer-BioNTech and Moderna vaccines, there was no difference in vaccine efficacy between the people who got their second dose on day 19 (two days before the 21-day recommendation for Pfizer’s vaccine) and the people who got it on day 42.

“Importantly, there was no decrease in protection between the first dose and the second dose,” the Journal reported.