So Yun Lim¹ Jiwon Jung^1,2 Ji Yeun Kim¹ Soonju Park³ Ji-Soo Kwon¹ So Yeon Park² Sun-Kyung Kim² Young-Ju Lim² Eun Ok Kim² Seongman Bae¹ Min Jae Kim¹ Yong Pil Chong¹ Sang-Oh Lee¹ Sang-Ho Choi¹ Yang Soo Kim¹ Nakyung Lee³ Kideok Kim³ David Shum³ Youngmee Jee³ Sung-Han Kim^1,2

¹Department of Infectious Diseases, Asan Medical Center, University of Ulsan College of Medicine, Seoul, Republic of Korea ²Office for Infection Control, Asan Medical Center, University of Ulsan College of Medicine, Seoul, South Korea ³Institut Pasteur Korea, Seongnam-si, Republic of Korea

So Yun Lim, Jiwon Jung, Ji Yeun Kim and Soonj Park have contributed equally to this work.

Correspondence Sung-Han Kim, Youngmee Jee

Dear Editor,

Breakthrough infection of SARS-CoV-2 has been increased according to the emergence of the delta variant and vaccine-induced waning immunity.1 In addition, epidemiologic studies have reported that people with previous SARS-CoV-2 infection show decreasing antibody levels against SARS-CoV-22 and can be reinfected with the virus later on.3 Here, we assessed the waning vaccine effectiveness by comparing the breakthrough infection rate between healthcare workers (HCWs) vaccinated with two doses of ChAdOx1 nCoV-19 (ChAdOx1) or mRNA vaccine. We also compared immune responses against ancestral SARS-CoV-2 and the delta variant between individuals with natural infection and HCWs who received the ChAdOx1 or BNT162b2 vaccine.