The major result of this modeling study is that gradual relaxation of NPIs, such as large-scale detection and quarantine, would be safe as vaccine coverage increased. In particular, for the transmission of wild strain in a city with a population of 20 million, NPIs can be relaxed when vaccine coverage reaches 50.42%. The outcomes will be improved if the vaccination strategy was accelerated or high-risk groups were given priority.
Consistent with previous studies [52–55], our results suggested that the relaxation of NPIs before establishment of sufficient immunity increased the probability of COVID-19 resurgence (maximum daily infected cases and the number of deaths). In particular, our model indicated that if vaccine effectiveness was 79.34%, vaccine coverage must be 50.42% before NPIs can be fully relaxed. Before vaccine coverage reaches 50.42%, NPIs still had a significant impact in preventing resurgence, and some NPIs were still needed even if vaccine coverage increased. However, once vaccine coverage reached 50.42%, almost all NPIs can be relaxed, and when coverage reached 75.62%, resurgence was very unlikely. We also estimated that 8 months are needed to achieve the vaccine coverage threshold for the fully relaxation of NPIs in the combination of accelerated vaccination strategy and key groups firstly strategy. However, if we conduct a slow vaccination strategy, NPIs would not be fully liberalized in three years. The vaccination coverage threshold 50.42% was estimated based on the transmissibility of wild strain and the vaccine efficacy against wild strain infection. Although there were some variants of SARA-CoV-2, such as the Alpha (B.1.1.7), Beta (B.1.351), Gamma (P.1) and Delta (B.1.617) variant, the protection of COVID-19 vaccine seems to retain against the disease . While some study found that the COVID-19 vaccine effectiveness against the Beta variant was lower [56–58]. As our results suggested that with the increasing of transmissibility of variants and the possible decreasing of vaccine effectiveness, the vaccine coverage threshold for safely relaxing NPIs would be higher. As scientists around the world learn more about the Delta variant of the novel coronavirus, they have been finding that it is more transmissible than the variants that have preceded it [51, 59], further implying the need of speeding up vaccination.
A localized COVID-19 outbreak occurred in Guangzhou on May 21, 2021 and on May 31, 2021, there were 34 symptomatic cases and 8 asymptomatic infections . According to public data, we estimated that vaccine coverage was 40% (effective vaccinated population: 31.74%) in Guangzhou on May 23, 2021. If Guangzhou implemented moderate NPIs, in particular if the MDT was 7 to 8 days, the probability of resurgence was 30–60%. This suggested that NPIs, such as the social distancing, large-scale nucleic acid testing, close contact tracking, and centralized isolation, still played a significant role in reducing the probability of resurgence and controlling local resurgences before the vaccine coverage threshold was attained. On 29 May 2021, there were 12 COVID-19 asymptomatic infections in Guangzhou , which had met the criteria of resurgence. However, even if Guangzhou lifted all NPIs, the resurgence probability was estimated about 90%. Those facts suggested that the virus might be more infectious, or the efficiency of vaccine may be not high enough for the variants. As vaccine coverage increased, the NPIs can be gradually relaxed without increasing the risk of resurgence.
The results of the present study suggested that acceleration of vaccination and targeting high-risk groups could reduce the probability of COVID-19 resurgence, especially when implemented early during the vaccination program. This is consistent with the results of previous studies [55, 61]. Accelerating vaccination is also necessary to prevent the transmission and spread of more contagious SARS-CoV-2 variants . When a high-risk group was given priority, an accelerated vaccination had a greater effect on reducing the number of cases, indicating that rapidly achieving high vaccine coverage was more important. High vaccine coverage and effectiveness provides long-lasting protection and greatly reduces the probability of resurgence . And compared with no vaccination, introducing vaccination had high cost-effectiveness .
Vaccine hesitancy is a complex public health issue, and obviously hinders vaccination programs. At the end of March 2020, when the first wave of the COVID-19 outbreak was controlled in China, 67.1 to 91.3% of people were willing to accept the available COVID-19 vaccine [62, 63]. However, in May 2020, 83.5% of people said they had the intent intended to get vaccinated in China, and only 28.7% reported they definitely intended to get vaccinated . Because of the successful control of the COVID-19 outbreak and the low incidence rate of COVID-19 in China, many people believed that vaccination was unnecessary . Our results suggested that there is a high probability of resurgence if the NPIs are relaxed before the target vaccine coverage is achieved. Therefore, to reduce vaccine hesitancy, it is necessary to educate the general public about the safety, benefits, and importance of vaccination. There are evidences that individuals at high-risk have greater acceptance of the vaccine . Our results indicated it is essential to improve vaccine coverage for these high-risk individuals as soon as possible to prevent resurgence.
There are some limitations of the current study. Our model did not consider the characteristics of the population, such as age, sex, and occupation. A heterogeneous population might influence vaccine coverage. Thus, a more sophisticated model, such as an agent-based model, is more suited for addressing the issue of population heterogeneity.
In conclusion, our study estimated that vaccine coverage of 50.42% was needed before NPIs can be fully relaxed. As vaccine coverage increases, the NPIs can be gradually relaxed. Until that threshold is reached, however, strict NPIs are still needed to contain the epidemic. An accelerated vaccination strategy was the most effective measure for preventing resurgence, followed by providing vaccination to high-risk groups. Targeting of high-risk groups for vaccination may be the best approach if there are insufficient vaccine doses. The more transmissible SARS-CoV-2 variant lead to higher resurgence probability, which indicates the importance of accelerated vaccination and achieving the vaccine coverage earlier.