Flexi Says: Controlling the spread of a virus depends on a safe and effective vaccine. Scientists estimate that it will take 18 months from the start of the Covid-19 pandemic to develop a vaccine. The spread of some viruses can be slowed once a large percentage of a population has been exposed to the virus and develops immunity. However, viruses that spread and mutate quickly, like the viruses which cause influenza and colds, are difficult to control because immunity to one strain does not guarantee immunity to new mutations. Scientists are unsure of how quickly the novel coronavirus will mutate or whether people who have had the virus will have long term immunity.