By Nikhila Natarajan New York, Jan 7 : At least 29 people vaccinated with mRNA-based Covid-19 shots in the US developed anaphylaxis - a "life-threatening" allergic reaction that comes on within minutes of receiving the intramuscular injection. A report on the CDC website lists 21 such cases but CDC officials clarified in a call with reporters that the number has been updated to 29 as on Wednesday. The median interval from vaccine receipt to symptom onset was 13 minutes, according to an update by the Centers For Disease Control and Prevention, America's premier public health agency that is returning to its pre-eminent role in health communications after months of political interference from the Donald Trump White House. The CDC is recommending that vaccine locations must screen for contraindications before the shots are given and must have supplies of epinephrine ready to treat people experiencing anaphylaxis symptoms. Around 80 per cent of patients who reported anaphylaxis symptoms had a documented history of allergies or allergic reactions, including to drugs or medical products, foods, and insect stings. By December 23, 2020, the CDC reported there were a total of 4,393 (0.2 per cent) "adverse events" among 18,93,360 first doses of Pfizer-BioNTech Covid-19. Vaccinations in America began on December 14. So far, two vaccines have received emergency approval from the US Food and Drug Administration -- Pfizer-BioNTech and Moderna. /IANS