The government on 16 June 2021 issued a clarification stating that Covaxin, the Covid-19 vaccine developed by Hyderabad-based Bharat Biotech, did not contain the serum of a newborn calf. This was in response to discussions on the social media about the presence of calf serum in Covaxin. In the clarification, the government reiterated the well-known use of calf serum, as also serum extracted from other animals, in the development of vaccines. These are needed to grow the disease-causing virus, bacteria or other pathogens in the laboratory, but do not themselves become an ingredient of the vaccine. Vaccines like the one made by Bharat Biotech uses the disease-causing virus itself to trigger an immune response in human beings. The virus is killed, or inactivated, before being used in the vaccine, and injected into the human body, but it is still able to trigger an immune response. To be used in the vaccine, the virus needs to be grown, or cultured, in the laboratory. Scientists try to create conditions conducive for the growth of these viruses by recreating the kind of environment that exists in an infected person’s tissues. Therefore, solutions containing ‘nutrients’ act as the growth medium for the virus. These nutrients, like specific sugar and salt molecules, are extracted from tissues of suitable animals like horses, cow, goat or sheep. The virus grows in these nutrient-rich solutions. After that, it goes through several stages of purification that make it suitable to be used in a vaccine. There is no trace of the growth medium after the entire process is over. According to the website of the Food and Drug Administration of the United States, cow components are used mainly because cows are large animals, easily available, and rich in some of the useful chemicals and enzymes. “Cow milk is a source of amino acids, and sugars such as galactose. Cow tallow derivatives used in vaccine manufacture include glycerol. Gelatin and some amino acids come from cow bones. Cow skeletal muscle is used to prepare broths used in certain complex media. Many difficult to grow micro-organisms and the cells that are used to propagate viruses require the addition of serum from blood to the growth media,” it says. Synthetic serum, and other nutrients, has also been developed, but according to one international manufacturer of these chemicals, Thermo Fisher Scientific, serum from newborn calf, which has been used in vaccine production for over 50 years, has “proven itself as an extremely effective growth supplement”. Historically, animal serum has been used in the development of vaccines in other ways as well. The use of horse serum as an antibody supplement in diphtheria vaccine is more than 100 years old. Horses used to be injected with small doses of bacteria that caused diphtheria so that they could develop antibodies. Later, the blood of the infected animal was used to extract the antibodies and used in the vaccine. The story of the Poonawalla family, which graduated from owning a horse-breeding farm that also used to supply horse serum used in vaccines, to setting up the company that is now the world’s largest producer of vaccines, Serum Institute of India, is very well known.