Here's how long you can wait for a second Covid-19 vaccine dose if the winter storms delay it

(CNN)Some vaccine providers have been forced to cancel Covid-19 vaccine appointments due to the winter weather that has ravaged much of the US and caused delays in vaccine deliveries.

That can be worrisome for people who were set to receive their second dose of the two-dose Covid-19 vaccines, which are supposed to have a second inoculation administered three or four weeks after the first.

But if you're one of those people, there's good news.

According to the US Centers for Disease Control and Prevention, the second dose of the vaccine can be administered up to 42 days, or six weeks, after the initial inoculation.

So if your appointment for a second dose was delayed or canceled due to winter weather, there should still be time to get fully vaccinated.

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Both vaccines on the US market -- developed by Pfizer-BioNTech and Moderna -- require two doses to reach about 95% efficacy, and the second doses were intended to be administered 21 days and 28 days after the first, respectively.

The CDC's website says the agency still recommends the second dose be administered "as close to the recommended interval as possible."

"However, if it is not feasible to adhere to the recommended interval and a delay in vaccination is unavoidable," the website says, "the second dose of Pfizer-BioNTech and Moderna COVID-19 vaccines may be administered up to 6 weeks (42 days) after the first dose."

The CDC is clear patients should not receive the second dose earlier than recommended, and there's still limited data on how effective the vaccines are if the second inoculation takes place beyond the six-week window.

At an event last month, Pfizer CEO Albert Bourla emphasized the importance of administering the second dose of his company's vaccine on time. But he doesn't think "giving it a week later or two is a very big issue."

"You need to make sure you give the second doses as the studies recommend the vaccine works, which is in three weeks," Bourla said during an event hosted by Bloomberg Media. "In our study we actually had from 19 to 42 (days). Within this framework, I'm fine. Beyond that, it's serious."

States see delayed vaccine shipments

The frigid weather has crippled large swathes of the country, and it's posed challenges for shipments of vaccines allocated as first and second doses.

Health officials in places like Dallas County, Texas, say they recognize the need for a timely second dose.

"We understand the urgency to administer second doses of the vaccine, but we must also balance people's safety," the county said in a news release this week. "As soon as we can safely open again, we will."

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In Miami-Dade County, Florida, officials this week said about 2,000 people would not receive their scheduled second dose of the vaccine due to weather related supply delays. A spokesperson for the mayor's office told CNN the second doses of the Pfizer-BioNTech vaccine scheduled to be administered Thursday had not arrived. Everyone who was impacted has been notified, according to the spokesperson.

Some health officials have indicated they would prioritize rescheduling appointments for the second dose moving forward. A news release this week from the Alabama Department of Public Health said officials would "assure the opportunity for people to get their second dose over the next two weeks," an effort that would include "extended clinic hours as staffing availability permits."

In Nevada, the Southern Nevada Health District said in a statement Wednesday it would have to reschedule people who had appointments for second doses of the Moderna vaccine by a week (second dose appointments for the Pfizer-BioNTech vaccine were going forward as scheduled). About 4,000 people were being contacted by officials to reschedule. Additionally, the health district said it would only administer second doses next week.

Shipping companies like UPS and FedEx told CNN they were working to make sure vaccines were delivered. FedEx said vaccine deliveries were getting priority, but "prolonged severe weather is continuing to impact much of the FedEx network."