On Thursday, the Centers for Diseases Control and Prevention (CDC) has recommended that people should take either a Pfizer-BioNTech or a Moderna shot as a booster, when available. The recommendation was signed by Rochelle Walensky, the director of the CDC.

The new recommendation stated that “mRNA vaccines” were “preferred over the Janssen Covid-19 vaccine” in order to prevent the spread of Covid-19 in adults who were 18 years of age and above.

Members of the agency’s Advisory Committee on Immunization Practices had voted in favor of the recommendation. Dr. Walensky released the recommendation a short while after this vote. The committee voted in favor as the Food and Drug Administration (FDA) had updated the risk of rare but potentially life-threatening blood clots that have been linked to the vaccine by Johnson & Johnson (J & J) called the Janssen vaccine in the U.S.

Later on Thursday, the CDC also said that the supply of mRNA vaccine in the U.S. was in abundance and that there were almost 100 million doses available for immediate use. It also said that similar recommendations had been made in countries such as the U.K. and Canada.

The CDC said that the Janssen vaccine would be available for those who were not able to or not willing to receive an mRNA vaccine. They would still have access to the vaccine from J & J.

At least 54 among the millions who received the J & J single dose shot in the U.S. had suffered from blood clots and all of them were hospitalized. Thirty six of them were in intensive care. Nine of them have died. A majority of those who got blood clots were women aged 30 to 49. However, the number of blood clots formed as a result of the vaccine are comparably much lower than the blood clots that can form due to a COVID-19 infection.