There is no guarantee that scientists will be able to develop an effective vaccine against Covid-19, the head of the World Health Organization (WHO) has said.Dr Tedros Ghebreyesus told the EU Parliament's health committee on Thursday that even if a vaccine was developed, it would probably not be available for at least a year."It would be very difficult to say for sure that we will have a vaccine," Dr Tedros told EU lawmakers via video link."We never had a vaccine for a coronavirus. So this will be, when discovered, hoping that it will be discovered, it will be the first one."Vaccines help the body's immune system to recognise and fight pathogens like viruses. They are considered one of the most effective ways to prevent diseases such as Covid-19 from spreading, as they help communities build up immunity.