WASHINGTON (BLOOMBERG) - Follow-up doses of the Covid-19 vaccines could be given up to six weeks later if it is not feasible to get them in the recommended interval, the Centres for Disease Control and Prevention (CDC) said, with the United States Food and Drug Administration also offering some flexibility for "modest delays".
The guidance posted in a Jan 21 update to the CDC website said a second dose should be administered as close to the recommended schedule as possible, either three weeks for the Pfizer-BioNTech vaccine or four weeks for the Moderna shot.
But if it is impossible to get the follow-up shot on time, the CDC says people may schedule it as long as six weeks, or 42 days, after their initial dose. There is "limited data on efficacy" of the vaccines beyond that interval, according to the guidance, but if the second dose is administered later, "there is no need to restart the series".
The FDA said slight delays should not affect the protection offered by the vaccine, after the agency resisted pressure earlier this month to stretch supply by adding time between the two shots.
Both vaccines authorised for emergency use in the US were cleared based on trials of two doses weeks apart. A grace period of four days ahead of schedule would be considered valid for a second dose, but people should not receive the second dose earlier than that.
The CDC made the change in response to feedback on earlier guidance, agency spokesman Kristen Nordlund said in an e-mail.
It is intended to provide flexibility in situations where patients cannot return on a specific date or their circumstances change, for example, if they enter or leave a nursing home, she said. The CDC does not want guidance "to be so rigid that it creates unintended barriers", she said.
Analysis of the Pfizer shot included some people who got the second dose as late as 42 days after the first, she said. The agency is not recommending the change to stretch the supply of doses "but rather to address feasibility issues", she said.
The CDC also reiterated that doses from the two vaccines are not interchangeable and people should get a second dose of the same product.
In "exceptional situations" when the initial vaccine is unknown or unavailable, "any available mRNA Covid-19 vaccine may be administered at a minimum interval of 28 days", the agency said.
The CDC said the Covid-19 vaccines generally should be administered alone, not simultaneously with other inoculations, such as for the flu.
The need for follow-up doses at specified intervals is one layer of complication in the national vaccination campaign. Shortages of doses and confusion over supply has led to long waits and frustration, even as the incoming Biden administration pledges to accelerate the delivery of vaccines.
"The FDA recognises that getting as many people as possible across the country fully immunised will help to curtail the spread of the virus that causes Covid-19 and should be a priority," the FDA said in a statement.
"Modest delays in the administration of the second dose, if absolutely necessary, would not be expected to decrease the protection conferred by the 2nd dose and are preferable to not completing the 2-dose series."
Then FDA Commissioner Stephen Hahn and Peter Marks, head of the agency's office that oversees vaccines, signed on to an earlier statement that said extending the time between shots had not been studied and "may ultimately be counterproductive to public health". While Hahn has left the agency, Marks, a career employee, remains.
Mr Biden's plan for combating the pandemic released Thursday includes some wiggle room for alternative dosing schedules. A single-dose Covid vaccine from Johnson & Johnson is in late-stage trials now, with data expected to be analysed in the weeks ahead.
"The federal government will explore dose-sparing strategies that have the potential to substantially expand vaccine supply, while maintaining a commitment to abiding by FDA recommendations," according to the plan.