COVID-19 vaccination in the mass vaccination center: clinical practice and effectiveness analysis

Coronavirus disease has spread in almost all countries, and the pandemic remains aggressively spreading. According to the World Health Organization (WHO), as of September 7, 2022, there were more than 600 million confirmed cases of COVID-19 worldwide, including more than 6.4 million deaths. COVID-19 is a public health problem of global concern, posing a severe threat to the environment, health and economic recovery. During the pandemic, countries adopted prevention strategies such as contact tracing, lockdowns, social distancing, sanitation, mask wearing, strengthened border control, and active monitoring to detect and isolate imported cases quickly. Recently, many countries relaxed prevention and control measures, but vaccination is still advocated. Since the COVID-19 epidemic, China has promoted epidemic prevention and control with a nationwide effort, achieving rapid control of outbreaks and economic recovery. From August 2021, China adopted a dynamic zero-COVID-19 strategy (“external prevention input, internal prevention rebound”), using precise measures to quickly cut transmission chains. Since January 8, 2023, COVID-19 management in China has been adjusted to Category B infectious diseases, with vaccination and personal protection still advocated. There is no specific antiviral drug fully effective against COVID-19, though some medications improve outcomes in severe patients. Mass vaccination is essential to control the virus, mitigate and control the pandemic, and create prerequisites for adjusting prevention and control strategies. China began nationwide COVID-19 vaccination in March 2021, establishing mass vaccination centers to increase coverage; as of September 7, 2022, over 3.4 billion doses had been administered in China. Different countries have used varied models for large-scale vaccination (e.g., temporary sites in Italy, academic health centers in the U.S.). Large-scale vaccination increases coverage rapidly and helps form community immunity. Clinical practice and effectiveness analysis of vaccination in mass centers can inform emergency responses to emerging infectious diseases. However, planning and implementing vaccination strategies face challenges, and operation of mass vaccination centers requires further study. This study aimed to evaluate the criteria established by mass vaccination centers in Chongqing, China, vaccination experience, and post-vaccination adverse events and opinions, to provide a reference for other countries.

Study setting: Nan'an District, Chongqing (area 262.43 km², population 1.2 million, urbanization rate 97.8%). The mass vaccination center was located at Jiangnan Sports Center, with capacity >3,500 people, and divided into Sites A and B. Each site included functional areas: entrance, waiting, pre-inspection/registration, inoculation, cold chain, observation, and rescue, with one-way flow to minimize cross-infection. Vaccination workflow, contraindications, procedures, and possible adverse reactions were displayed prominently.

Equipment and facilities: The sports center was transformed to include all functional areas equipped for epidemic prevention/control, cold chain (medical refrigerators, monitoring devices, thermometers, temperature records), inoculation (computers, scanners, printers, disinfectants, syringes, treatment trays, waste management), observation/rescue (first-aid medicines and equipment including epinephrine, defibrillator/AED, monitors, oxygen, airway equipment, etc.), and entrance/waiting/registration area equipment (thermometers, disinfectants, seating, heating/cooling, information materials).

Personnel and training: Staffing included a general leader (1), pre-check/registration personnel (12), vaccinators (10), medical rescuers (2), network maintenance (1), quality control officers (2), and several volunteers for order maintenance. Staff allocation was adjusted based on daily appointment numbers. The Nan'an CDC provided special training on emergency use of COVID-19 vaccines and on managing suspected vaccination reactions.

Vaccines and schedules: Two vaccine types were used—(a) inactivated COVID-19 vaccines (Vero cells) by Sinopharm and Sinovac/CoronaVac: 2 doses (0.5 mL) IM, second dose 21 days after first, for people ≥3 years; (b) recombinant protein (CHO cell) vaccine by Anhui Zhifei Longcom: 3 doses (0.5 mL) IM at monthly intervals, for people ≥18 years.

Implementation phases: Vaccination prioritized in phases: (1) highest-risk workers (e.g., cold chain, ports, transport, healthcare, markets, outbound travelers to medium/high-risk areas); (2) adults 18–59 years without contraindications; (3) people ≥60 years and 12–17 years without contraindications; (4) children 3–11 years without contraindications.

AEFI surveillance and reporting: Any reaction suspected related to vaccination should be reported, especially severe or cluster AEFI. Routine online reporting was not required for expected minor reactions unless fever ≥38.5°C or local reactions (redness/swelling/induration) >2.5 cm. Healthcare workers could detect AEFI within 30 minutes post-vaccination; recipients/guardians were encouraged to report thereafter. Reportable AEFI must be entered into China’s disease prevention and control information system within 48 hours; suspected severe events (death, severe disability, clusters) within 2 hours. The Nan'an CDC verified and classified AEFI (case investigation for severe AEFI; generally unnecessary for non-serious).

Infection prevention and control: To prevent nucleic acid contamination and cross-infection: standardized PPE use (protective clothing, masks, face shields, gloves, boot covers) for inoculation nurses; environmental disinfection before and after daily vaccination using chlorine-containing disinfectant (250–500 mg/L) or 1% hydrogen peroxide with 20–30 min contact time; strict medical waste management per regulations, centralized collection and disposal; prohibition on discarding or removing used cotton swabs/balls from site.

Statistical analysis: Data were analyzed using Microsoft Excel 2019 and SPSS v26.0; process flows were diagrammed using an online tool (processon.com). Risk of AEFI was estimated by age, sex, and vaccine type; p<0.05 considered statistically significant.

Vaccination process: At entrance, staff screened for epidemiological history and fever; only those without could proceed. After waiting, recipients completed pre-exam and signed informed consent, then received vaccination at stations equipped with desktop refrigerators. Post-vaccination, recipients were observed for 30 minutes in an observation area with at least two medical personnel. A first-aid room and ambulance were available; allergic reactions were treated and transferred to hospital as needed.

• Volume and distribution: From March 26, 2021 to April 28, 2022, 381,364 doses were administered at the Nan'an District mass vaccination center. By vaccine: CoronaVac (Vero cell) 159,047 doses; Sinopharm (Vero cell) 130,118 doses; Recombinant protein (CHO cell) 92,199 doses.
• Throughput dynamics: Maximum daily doses reached 7,550 on July 1, 2021, which was 5.46 times the average at other vaccination sites. Daily volumes varied widely (0 to >7,500), driven by local policies and vaccine availability; staffing and station numbers were adjusted accordingly.
• Safety (AEFI): Only four AEFI were reported (incidence 1.04 per 100,000 doses), all non-severe: injection site redness (2 cases, CHO), fever (1 case, Vero), and anaphylactic rash (1 case, CHO). No severe AEFI occurred.
• Risk factors for AEFI: No significant associations with sex (p=0.617) or age group (<60 vs ≥60 years; p=0.308). Initially, CHO vaccine showed higher AEFI tendency than Vero (p=0.046). After excluding recipients <18 years, the difference was not statistically significant (p=0.066).
• Operational effectiveness: The mass vaccination center operated safely and effectively, providing high-volume services and contributing to increased vaccination rates in the district.

The study demonstrates that a well-designed, centrally located mass vaccination center with clear workflows, adequate staffing and training, robust cold-chain and emergency preparedness, and strict infection control can safely and efficiently deliver large vaccination volumes. The very low AEFI rate and absence of severe events address public concerns about vaccine safety and support ongoing vaccination efforts, including among older adults. The lack of significant associations between AEFI and age or sex aligns with other reports and supports broad eligibility. While an initial signal suggested higher AEFI with the CHO vaccine compared with inactivated Vero vaccines, this was not significant after age restriction, and the small number of events limits inference. Operationally, coordination among government, health authorities, community workers, and volunteers was critical. Site selection with convenient access and measures to prevent cross-infection (screening, masking, disinfection, temporary closures during outbreaks) ensured safety. The center complemented routine clinics and provided tailored services (e.g., on-site vaccination for those with mobility issues). These findings support mass vaccination centers as key infrastructure for rapid coverage expansion and as a model for future emergency immunization campaigns.

The mass vaccination center in Nan'an District operated successfully, delivering large numbers of COVID-19 vaccinations safely and efficiently, with a very low incidence of AEFI and no severe events. This operational model provides a practical framework and reference for other countries and regions planning mass vaccination efforts for COVID-19 or future emerging infectious diseases.

• High staff turnover required time for new personnel to become familiar with responsibilities, potentially causing omissions.
• Inaccurate daily demand forecasting at times reduced vaccination efficiency.
• Underreporting of AEFI is likely due to reliance on voluntary reporting; recipients may not report common reactions such as injection pain or mild fever. These issues should be addressed in future practice to improve monitoring and efficiency.