Under an existing agreement, almost 150 million doses of the AstraZeneca/Oxford vaccine candidate are ready to be shipped out to countries in need upon completion of the WHO's prequalification process to confirm the vaccine's safety and efficacy.
The new guidance stresses that "every effort" should be taken to ensure that a patient receives the same vaccine, but that "any available mRNA COVID-19 vaccine may be administered at a minimum interval of 28 days between doses" in certain unique circumstances such as available supplies being are limited or the patient being unaware of which vaccine they originally received. But if only the opposite type of vaccine is available, the CDC said that the other vaccine can be administered and is considered effective.
Most of the cases involved people with a history of allergic reactions.
Following authorizations from the Food and Drug Administration (FDA) and recommendations from the Centers for Disease Control and Protection (CDC), Pfizer began distributing the vaccine to be administered by health care professionals all across the country.
The Pfizer vaccine, jointly developed with German partner BioNTech SE, was the first vaccine to receive emergency use validation from the World Health Organization since the outbreak of the novel coronavirus began.
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Pfizer, which developed the vaccine with its German partner BioNTech, has reduced the volume of the vaccine it will deliver to European Union countries this week.
"The clinical and epidemiologic characteristics of anaphylaxis case reports after receipt of Moderna Covid-19 vaccine are similar to those reported after receipt of the Pfizer-BioNTech Covid-19 vaccine", the report said. The majority of the 100 million doses are earmarked for delivery in the first quarter of the year.
A total of 1,266 patients receiving their first Moderna dose experienced "adverse events", such as a rash or respiratory problems, the agency said.
Asked about how many vaccine doses all manufacturers might be able to produce this year, Gavi chief executive Seth Berkley said: "I think we're talking about numbers in the range of six to seven billion doses".
"But we can only end the pandemic anywhere if we end it everywhere".