Pfizer plans to ask US regulators to authorize a booster dose of its Covid-19 vaccine within the next month, the drugmaker’s top scientist said on Thursday. The announcement was based on evidence of greater risk of reinfection six months after inoculation and due to the spread of the highly contagious Delta variant of coronavirus. And the US pharmaceutical company and its German partner BioNTech have started designing a version of their vaccine specifically to combat the highly-contagious Delta variant, Pfizer’s chief scientific officer, Mikael Dolsten, said. However, the companies do not think they will need to replace the current version of their highly-successful shot. Dolsten said the recently reported dip in the vaccine’s effectiveness in Israel was mostly due to infections in people who had been vaccinated in January or February. The country’s health ministry said vaccine effectiveness in preventing both infection and symptomatic disease fell to 64% in June. “The Pfizer vaccine is highly active against the Delta variant,” Dolsten said in an interview. But after six months, he said, “there likely is the risk of reinfection as antibodies, as predicted, wane”. Pfizer did not release the full set of Israeli data on Thursday, but said it would be published soon. He stressed that data from Israel and Britain suggests that even with waning antibody levels, the vaccine remains around 95% effective against severe disease. The vaccine, developed with a German partner, BioNTech SE , showed 95% efficacy in preventing symptomatic Covid-19 in a clinical trial the companies ran last year. Dolsten said that early data from the company’s own studies shows that a third booster dose generates antibody levels that are five to 10-fold higher than after the second dose, suggesting that a third dose will offer promising protection. He said that multiple countries in Europe and elsewhere have already approached Pfizer to discuss booster doses, and that some may begin administering them before a potential US authorization. Dolsten said he believed that booster shots were particularly important in older age groups. Pfizer’s chief executive, Albert Bourla, has said people will probably need a booster dose of the company’s vaccine every 12 months – similar to an annual vaccination against influenza. But some scientists question when, or whether, such shots will be needed.