Giesecke and Tegnell believed herd immunity would arrive quickly. In the Lancet article, Giesecke claimed about 21% of residents of Stockholm county had already been infected by the end of April; Tegnell predicted 40% of them would have antibodies by the end of May. When initial studies showed the number was actually about 6% in late May, Tegnell said immunity was hard to measure. FoHM continued to say Swedes had built up immunity, but in September it backtracked, estimating that “just under 12%” of Stockholm residents, and 6% to 8% of the Swedish population as a whole, had antibodies to the virus by mid-June