In the absence of effective control measures for COVID-19 [2], Shanghai’s high population density and general susceptibility of its residents makes it vulnerable to a renewed threat from COVID-19. In this study, we found that willingness to vaccinate against COVID-19 among Shanghai residents was high (86.8%). The proportion of participants willing to receive COVID-19 vaccines in our study was similar to that observed in two surveys conducted in Chile (90.6%) [13] and Australia (85.8%) [14], but higher than that observed in seven European countries (73.9%) [15] and in France (74%) [16]. However, willingness to accept COVID-19 vaccines in these surveys was substantially higher than willingness to vaccinate against H1N1 during the 2009 pandemic (8.7–67%) [17]. Most participants who were willing to be vaccinated believed that vaccination could reduce the risk or psychological burden of COVID-19 infection, as well as the likelihood of serious illness should infection occur. Because of the high risk of infection and high burden of COVID-19, there is strong willingness to vaccinate against COVID-19, although the severity of the COVID-19 epidemic varies among countries.
The herd immunity threshold, which describes the proportion of the population that needs to be immune to contain transmission, depends on the basic reproductive number (R0) of the disease [15]. Studies have reported values of R0 for SARS-CoV-2 ranging from 2.2 to 5.7 [18–20], and thus coverage would need to reach 54.5–82.5% for effective herd immunity [15]. Although willingness to vaccinate observed in this study was higher than the herd immunity threshold (82.5%), this is only an estimate of willingness to vaccinate and the actual coverage may be overestimated. Several studies showed that willingness to receive H1N1 vaccines was higher than the actual vaccination coverage during the 2009 H1N1 pandemic [21]. However, because H1N1 did not cause social consequences with the same magnitude as COVID-19 [22] and levels of awareness regarding the dangers of COVID-19 are likely much higher compared with H1N1, actual vaccination coverage may be increased. The results of a modeling study showed that coverage may be determined by the effectiveness of COVID-19 vaccines. As long as the effectiveness of the vaccine reaches 60–70%, 50–70% vaccination coverage can control the COVID-19 outbreak [23]. Because the safety and efficacy of a vaccine can greatly influence participants' willingness to be vaccinated, the development of an efficient and safe COVID-19 vaccine is an urgent goal.
In this study, participants were more reluctant to accept vaccinations on behalf of older individuals in their households. However, the risk of serious disease and death following COVID-19 infection is higher in older individuals [24, 25]. Data from the United States indicated that 31% of cases, 45% of hospitalizations, 53% of intensive care unit admissions, and 80% of deaths associated with COVID-19 occurred among adults aged ≥ 65 years [25]. During the COVID-19 epidemic, family clusters were the main modes of human-human transmission accounting for 57.6% of all cases [26]. We found that participants with older individuals in their homes were less willing to vaccinate themselves and their children. In addition, participants with no self-reported history of influenza vaccination were less likely to accept COVID-19 vaccines for themselves, their children, and older individuals in their households. However, influenza vaccination coverage among Chinese residents was only 1.5–2.2% from 2004 to 2014, much lower than the proportion of participants’ self-reporting influenza vaccination history in our study [27]. Therefore, increasing the coverage of COVID-19 vaccines not only among older individuals, but also among their family members, will be required to prevent transmission within the family.
We also found that participants with healthcare-related occupations were more reluctant to have their children vaccinated against COVID-19. Furthermore, participants with higher levels of education were less likely to accept COVID-19 vaccines for their children. Children are less likely to have severe symptoms of COVID-19 infection [25]. However, children are at similar risk of infection as the general population, and mild or asymptomatic cases among children may also be sources of SARS-CoV-2 transmission [28]. Therefore, governments need to ensure that COVID-19 vaccination coverage in children is maintained at a high level.
According to the Law of the People's Republic of China on Vaccine Administration [29], a vaccine can be used on an emergency basis within a certain scope and time limit, with approval of governments, following a particularly significant public health event. An official from China's health ministry said on television on 22 August 2020 [30] that China had initiated the emergency use of COVID-19 vaccines since 22 July. The purpose of emergency use is to ensure the stable operation of the city in the event of another COVID-19 epidemic by first vaccinating specific groups such as medical, epidemic prevention and border control personnel as well as personnel responsible for the basic operations of the city. Although there are several COVID-19 vaccines entering phase 3 clinical trials in China, approved vaccines may face initial undersupply challenges because of limitations in production capacity. To this end, Henn argues that in the absence of adequate supplies of future COVID-19 vaccines, the vaccine should be provided first to physicians and nurses as well as to police and other public security officers; second to organ transplant recipients; and finally to all others in order of date of birth from old to young, without exceptions [31]. This mirrors the vaccination strategy used in response to the 2009 H1N1 pandemic in China, in which priority groups (e.g., older individuals, students, civil servants, etc.) received the vaccine followed by other groups [32]. However, according to modeling results, the priority of targeted vaccination would depend on the effectiveness of future COVID-19 vaccines [23].
A limitation of this study was that the subjects were all from Shanghai, where the number of COVID-19 cases was small. Thus, our findings may not be fully generalizable to other regions. However, even among residents of Wuhan, the epicenter of the pandemic, only 2% had detectable IgM/IgG antibodies against SARS-CoV-2 [33]. A strength of our study was that in addition to disclosing their own willingness to vaccinate, participants responded on behalf of any children or older individuals living with them. Our study contributes novel and timely evidence to better understand the need for future COVID-19 vaccines following the COVID-19 epidemic. There are very limited data on this topic.