New York, July 24 : The US government has purchased an additional 200 million doses of the Pfizer-BioNTech Covid-19 vaccine. This brings the total number of doses to be supplied by Pfizer and BioNTech to the US government under its existing supply agreement to 500 million. About 110 million of the additional doses will be delivered by December 31, 2021, while the remaining 90 million doses will be delivered by April 30, 2022, Pfizer and BioNTech announced in a statement on Friday. The US government also has the option to acquire an updated version of the vaccine to address potential variants as well as new formulations of the vaccine, if available and authorised, it said. The additional vaccines could be used to inoculate children under 12 and provide booster doses to the "immunocompromised" people in the US. Early this week, US President Joe Biden had said that children under the age of 12 could "soon" be eligible to receive a Covid 19 vaccine, predicting that his government could green light the rollout for young Americans in the next few months. The vaccines for children could be ready by August or later in the fall -- a time many schools across the US plan to reopen, the CNN reported. Amid surging infections from the Delta variant, the US Centers for Disease Control and Prevention (CDC) is also considering to incolate all "immunocompromised" people which include organ transplant recipients, people on cancer treatments, and those living with rheumatologic conditions, HIV and leukaemia, with additional doses of the Covid vaccines. "As a long-term partner to the US government in the fight against this pandemic, we are proud of the impact of vaccination efforts across the country. Vaccines have been and will remain critical to protecting lives against this devastating disease," said Albert Bourla, Chairman and Chief Executive Officer, Pfizer, in a statement on Thursday. "These additional doses will help the US government ensure broad vaccine access into next year," he added Last month, Pfizer and BioNTech had shared plans to provide the US government with 500 million doses of the companies' Covid-19 vaccine for donation to the world's poorest nations. Roughly 86 million people have been fully vaccinated with the Pfizer-BioNTech vaccine in the US as of Thursday, according to the US Centers for Disease Control and Prevention. The Pfizer-BioNTech Covid-19 vaccine has been authorised for emergency use by the US Food and Drug Administration (FDA), under an Emergency Use Authorisation (EUA) in individuals 12 years of age and older. Pfizer is also conducting clinical trials to evaluate the safety and efficacy of the vaccines in children under 12. The results for children aged 5 to 11 are expected in September, after which the company aims to ask the FDA to authorise emergency use. Results of the trial of the Covid jabs for children aged two to five-year-olds will likely come in October or November. /IANS