Effectiveness of inactivated SARS-CoV-2 vaccines against the Delta variant infection in Guangzhou: a test-negative case-control real-world study

Emerg Microbes Infect. 2021 Dec;10(1):1751-1759. doi: 10.1080/22221751.2021.1969291.


The effectiveness of inactivated SARS-CoV-2 vaccines against the Delta variant, which has been associated with greater transmissibility and virulence, remains unclear. We conducted a test-negative case-control study to explore the vaccine effectiveness (VE) in real-world settings. We recruited participants aged 18-59 years who consisted of SARS-CoV-2 test-positive cases (n = 74) and test-negative controls (n = 292) during the outbreak of the Delta variant in May 2021 in Guangzhou city, China. Vaccination status was compared to estimate The VE of SARS-CoV-2 inactivated vaccines. A single dose of inactivated SARS-CoV-2 vaccine yielded the VE of only 13.8%. After adjusting for age and sex, the overall VE for two-dose vaccination was 59.0% (95% confidence interval: 16.0% to 81.6%) against coronavirus disease 2019 (COVID-19) and 70.2% (95% confidence interval: 29.6-89.3%) against moderate COVID-19 and 100% against severe COVID-19 which might be overestimated due to the small sample size. The VE of two-dose vaccination against COVID-19 reached 72.5% among participants aged 40-59 years, and was higher in females than in males against COVID-19 and moderate diseases. While single dose vaccination was not sufficiently protective, the two-dose dosing scheme of the inactivated vaccines was effective against the Delta variant infection in real-world settings, with the estimated efficacy exceeding the World Health Organization minimal threshold of 50%.

Keywords: Coronavirus disease 2019; Delta variant; SARS-CoV-2; efficacy of vaccination; vaccine.

MeSH terms

  • Adolescent
  • Adult
  • Age Distribution
  • COVID-19 / classification
  • COVID-19 / prevention & control*
  • COVID-19 Vaccines / administration & dosage
  • COVID-19 Vaccines / standards*
  • Case-Control Studies
  • China
  • Disease Outbreaks
  • Female
  • Genetic Variation
  • Humans
  • Male
  • Middle Aged
  • SARS-CoV-2 / genetics*
  • Vaccines, Inactivated / administration & dosage
  • Vaccines, Inactivated / standards
  • Young Adult


  • COVID-19 Vaccines
  • Vaccines, Inactivated

Grant support

This work was supported by The Key Project of Medicine Discipline of Guangzhou [grant number 2021-2023-11]; The Basic Research Project of Key Laboratory of Guangzhou [grant number 202102100001]; Foshan Scientific and Technological Key Project for COVID-19 [grant number 2020001000430]; Zhuhai Scientific and Technological Key Project for COVID-19 [grant number ZH22036302200008PWC]; COVID-19 Vaccines Scientific Research Project of the China National Key R&D Programme “Post-marketing Research on Immunity Persistence and Effectiveness of Inactivated COVID-19 Vaccine” [grant number 2021YFC0863900]; The Health Science and Technology Project of Guangzhou “Research on immunization Strategy of SARS-CoV-2 vaccines” [grant number 20211A011051].