Preplanned Studies: Preliminary Study of the Protectiveness of Vaccination Against the COVID-19 in the Outbreak of VOC Omicron BA.2 — Jilin City, Jilin Province, China, March 3–April 12, 2022

An outbreak of coronavirus disease 2019 (COVID-19) caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Omicron BA.2 strain occurred in Jilin City, Jilin Province, China. By April 12, a total of 27,036 cases have been diagnosed by the reverse transcription-polymerase chain reaction (RT-PCR) as SARS-CoV-2 and reported via the internet-based national direct reporting system for infectious disease. The clinical severity of those cases was typed according to the Diagnosis and Treatment Protocol for COVID-19 Patient (9th edition) issued by the National Health Commission (NHC) (1). Among them, 13,164 cases were asymptomatic, 13,629 were mild, 196 were moderate, 32 were severe, and 13 were critical. A total of 241 cases were diagnosed as COVID-19, accounting for 0.89% of all COVID-19 cases. There were 2 fatal SARS-CoV-2 positive cases reported who died from other diseases. The SARS-CoV-2 Omicron BA.2 strain shows a great impact on transmission and immunity globally. Numerous studies have revealed that the vaccine effectiveness on COVID-19 transmission of Omicron strains is markedly reduced, regardless of the types of vaccines (2-5). In this report, the vaccine protectiveness on COVID-19 induced pneumonia and more severe clinical types of 241 cases in Jilin City were preliminarily analyzed.

Jilin City is the 2nd largest city in Jilin Province with a registered population of 3,623,713. Among them, 25.6% (915,090) were 60 years and/or older. The COVID-19 vaccination program has been implemented in Jilin City since the beginning of 2021. All people were vaccinated with domestic inactivated vaccines. By March 3, the coverages of COVID-19 full (2 doses) and booster (3 doses) vaccination in the general population were 81.1% (2,937,342) and 32.1% (1,163,069), while those in the population ≥60 years were 79.5% (727,950) and 29.8% (272,943), respectively. In addition, there were 68,347 people who received 1 dose vaccine but had not finished the full vaccination by March 3.

The general population, the population ≥60 years, and <60 years in Jilin City was divided into 3 groups, unvaccinated plus received 1 dose (0+1 dose), full vaccination (2 doses), and booster vaccination (3 doses). More COVID-19 cases distributed in the group of 0+1 dose (n=121) than that of 2 doses (n=90) and 3 doses (n=30). Among them, 2 moderate cases and 1 severe case received only 1 dose of the vaccine. The distribution of the clinical severity in the different groups of ages and vaccinations was summarized in Table 1. In general population, higher ratios of severe (21.5%) and critical (7.4%) cases were in the group of 0+1 dose, whilst lower ratios were in the groups of 2 doses (5.6% of severe and 4.4% of critical) and 3 doses (3.3% of severe and 0% of critical). Among those 241 cases, 127 cases were at ages of ≥60 years and 117 were <60 years. Markedly more numbers and higher ratios of critical cases were in the group of ≥60 years. Only 1 case <60 years displayed a critical phenotype and did not receive vaccination. There were no critical cases in the group of <60 years who received 2 doses of vaccination, and no severe or critical cases who received 3 doses.

The incidences (1/100,000) of COVID-19 in different vaccination groups were further calculated. For the general population in Jilin City, the incidences of moderate, severe, and critical cases in the group of 0+1 dose were 12.53, 4.08, and 1.31 per 100,000, respectively, which were 1.82-, 9.49-, and 3.85-fold higher than those in the group of 2 doses, and 5.03- and 44.47-fold higher than the moderate and severe cases that received 3 doses of vaccination (Figure 1A). For the population ≥60 years, the incidences of moderate, severe, and critical cases in the group of 0+1 dose were 29.92, 9.62, and 4.27 per 100,000, respectively, which were remarkably higher than that of the general population. The incidences of moderate, severe, and critical cases in the group of 0+1 dose were 4.13-, 43.72-, and 4.85-fold higher than those in the group of 2 doses, 13.28- and 22.37-fold higher than moderate and severe ones in the group of 3 doses, respectively (Figure 1B). Contrarily, the incidence of COVID-19 in the population <60 years of 0+1 dose group was lower, showing 9.82-, 1.60, and 0.20 per 100,000 in moderate, severe, and critical phenotypes, respectively. There were no critical cases in the group of 2 doses and no severe and critical cases in that of 3 doses. The incidences of moderate and severe disease of 0+1 group were 3.70- and 4.85-fold higher than that of patients receiving 2 doses, while the incidence of moderate cases was 3.81-fold higher than that receiving 3 doses (Figure 1C). The incidence of each type of COVID-19 in the population who were fully vaccinated or booster vaccinated in Jilin City is significantly lower than those who were unvaccinated and/or partially vaccinated. Booster vaccination induces stronger protectiveness for COVID-19 both in the population ≥60 years and <60 years.

Figure 1. 

The incidences of COVID-19 in the groups of unvaccinated and partially vaccinated (0+1 dose), fully vaccinated (2 doses), and booster vaccinated (3 doses). (A) In the whole population; (B) the population $ \ge $60 years; (C) the population <60 years.

Note: the X-fold changes in the incidence between groups are indicated. Abbreviation: COVID-19=coronavirus disease 2019. * Uncalculatable as there are no cases in the group to be compared.