By Jennifer Rigby and Francesco Guarascio LONDON (Reuters) – Leaders of the global scheme aiming to get COVID-19 vaccines to the world’s poorest are pushing manufacturers including Pfizer and Moderna to cut or slow deliveries of about half a billion shots so doses are not wasted. COVAX, the World Health Organization-led scheme, wants between 400 and 600 million fewer vaccines doses than initially contracted from six pharmaceutical companies, according to internal documents seen by Reuters. While at first the initiative struggled for shots as wealthy nations snapped up limited supply, donations…