Clinical trials of a Covid-19 vaccine have revealed a new formula is 94.5 per cent effective at preventing the virus.Biotech company Moderna has tested the vaccine on 30,000 volunteers. The news comes a week after Pfizer and BioNTech said their vaccine was more than 90 per cent effective.Dr Stephen Hoge, the company's president, says it's been a "long hard slog" to reach this point.US correspondent Simon Marks speaks to Susie Ferguson.