Dec 21 (Reuters) - The EU drug regulator said on Tuesday that it had not established a link between changes in menstrual cycles and COVID-19 vaccines, after a study in Norway suggested some women had heavier periods after being inoculated. The Norwegian Institute of Public Health (FHI) study released on Tuesday asked almost 6,000 women aged 18-30 about their menstrual cycles and bleeding patterns before and after vaccination. The article has not yet been peer-reviewed. The early results showed that 7.6% reported heavier periods before the first vaccination, rising to 13.6% after; and 8.2% before the second vaccine, which rose to 15.3% after the second shot. "Further studies will be needed where we measure hormone levels etc to fully determine that," Georgy Genov, head of pharmacovigilance at the European Medicines Agency, told a media briefing. He said it was important to note that any menstrual disorders seen in emerging studies had been transient. The Norwegian institute said young women who experienced heavy and persistent menstrual bleeding after receiving a COVID-19 vaccine may need to delay getting another shot until the cause is investigated and the symptoms have ceased. They should also consult their doctor to rule out other diseases that may require treatment. With any temporary menstrual changes in a regular cycle, the next vaccine dose can be given as planned. Still, the FHI said, changes were temporary side effects for most and should not prevent women from getting inoculated. "Vaccination gives protection to the individual against a severe COVID-19 course, as well as contributing to reduced transmission in society," it said. The EMA"s Genov said the agency had no data showing that any vaccines, including COVID-19 shots, affect people"s fertility.