Vietnam had the lowest global infection rate during the early phases of the SARS-CoV-2 pandemic. Although the early low infection rate was attributed to efforts in movement control orders (MCO) in the country, the later nationwide outbreaks of the delta and omicron strains despite similar MCO efforts, imply other contributing factors.
Our study is the first large-scale study to provide evidence on existing cross-reactive immunity against recent SARS-CoV-2 strains in Vietnam, at least 5 years prior to the COVID-19 pandemic, and to determine the extent of pre-existing immunity in association with emerging variants.
In this study, pre-COVID-19 samples demonstrated neutralizing activities against the Wuhan and alpha strains, but not against other variants of concern (VOCs: beta, gamma, and delta strains).
Preexisting humoral and cellular immunity recognize coronavirus (hCoV) epitopes, suggesting potentially that pre-COVID-19 immunity against SARS-CoV-2 is due to exposure to hCOV. Pre-existing antibodies demonstrated limited neutralizing capacity against newer SARS-CoV-2 variants and limited cellular immunity against SARS-CoV-2, suggesting to an extent, the occurrence of pre-pandemic immunity against SARS-CoV-2 but with less capability to address newer VOCs.
Overall, our study supports the hypothesis that factors, including SARS-CoV-2 pathogen transmissibility and pre-existing immunity, are associated with the transmission patterns in Vietnam region. The results, which demonstrated that pre-COVID-19 samples exhibited neutralizing antibodies against the Wuhan and alpha strains, imply that preexisting neutralizing antibodies are potentially important factors influencing the low SARS-CoV-2 transmission during the early phase of the COVID-19 pandemic in Vietnam.

Reference paper:
Nguyen TTN, Choo EM, Nakamura Y, Suzuki R, Shiina T, Shin-I T, Fukuta M, Nguyen CT, Nguyen TTT, Nguyen LKH, Hoang VMP, Morita K, Dang DA, Hasebe F, Le TQM, Moi ML. Pre-existing cross-reactive neutralizing activity against SARS-CoV-2 and seasonal coronaviruses prior to the COVID-19 pandemic (2014-2019) with limited immunity against recent emerging SARS-CoV-2 variants, Vietnam. Int J Infect Dis. 2024 Feb;139:109-117. doi: 10.1016/j.ijid.2023.11.008. Epub 2023 Nov 19. PMID: 37984763.
Research Paper

Collaborators:
● National Institute of Hygiene and Epidemiology, Hanoi, Vietnam
● Institute of Tropical Medicine, Nagasaki University, Nagasaki, Japan
● Tokai University School of Medicine, Kanagawa, Japan
● Repertoire Genesis Inc., Ibaraki, Japan
● BITS Co. Ltd., Tokyo, Japan

Funding
Japan Agency for Medical Research and Development (AMED) 233fa627001h0002, JP23wm0125006