Dec 23 (Reuters) - COVID-19 test maker Quidel Corp (QDEL.O) agreed to buy Ortho Clinical Diagnostics Holdings (OCDX.O) for about $6 billion in a cash and stock deal, less than a year since the Carlyle-backed firm went public. Quidel said on Thursday it would pay $24.68 per share, representing a premium of 24.7% to Ortho Clinical"s closing price on Wednesday, and would also acquire the company"s existing net debt of $2 billion. Ortho Clinical"s shares rose nearly 7% to $21.17, while Quidel fell 16% to $139.72. The deal will add Ortho Clinical"s COVID-19 antigen and antibody tests to Quidel"s own range of tests, including antigen and PCR for coronavirus. Quidel, which has benefited from high demand for these tests in the past two years, also sells molecular diagnostics and cell culture tests. Demand for COVID-19 testing has received a fresh boost from the fast-spreading Omicron variant in the United States, with pharmacy chains including Walgreens Boots Alliance (WBA.O) and CVS Health (CVS.N) as well as retailer Walmart (WMT.N) limiting sales of at-home tests. The variant has become the dominant strain in the country, accounting for 73% of infections for the week ended Dec. 18. Quidel, on a call with analysts, said it expects its Savanna PCR test platform to benefit from Ortho Clinical"s global presence. Quidel recently launched the test, which can detect two types of influenza and COVID-19, among others, in Europe. Ortho Clinical also makes products for diagnosing diseases and other equipment used for in-vitro diagnostics, a nearly $80 billion market that serves hospitals and clinics. The deal, expected to close in the first half of 2022, will deliver estimated cost savings of $90 million by the end of year three after its completion, the companies said. Perella Weinberg Partners LP and Citi served as Quidel"s financial advisers on the deal, and J.P. Morgan Securities for Ortho.