A single dose of Johnson & Johnson's experimental Covid-19 vaccine produced a strong immune response against the novel coronavirus in an early-to-mid stage clinical trial, according to interim results published on Friday, news agency Reuters reported. The vaccine was equally well-tolerated at two different doses, the results showed. A single shot vaccine being developed by Johnson & Johnson, versus a rival two-dose approach being tested by Moderna and Pfizer, could simplify mass distribution of the vaccine.