A technical report recently published by the European Center for Disease Prevention and Control (ECDC) has summarized all available literature regarding the risk of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) transmission from asymptomatic reinfected individuals with a history of SARS-CoV-2 infection or vaccination. The report also includes available information about the duration and characteristics of host immunity in response to natural SARS-CoV-2 infection or vaccination. The report is currently available on the ECDC website.
Background
Within one year of its emergence, the coronavirus disease 2019 (COVID-19) pandemic has infected more than 128 million individuals and claimed 2.8 million lives globally. Although the COVID-19 related mortality rate has reduced considerably in the later phase of the pandemic, there is a growing pool of evidence highlighting the risk of reinfection among COVID-19 recovered individuals. In this context, studies have shown that antibody-mediated immunity induced in response to natural SARS-CoV-2 infection does not persist for a long time, explaining the risk of reinfection.
Regarding COVID-19 prophylactic measures, several potential vaccines have recently been approved for emergency use in many countries. Clinical trials conducted to evaluate these vaccines have documented their effectiveness in preventing COVID-19 progression and reducing disease severity and mortality. However, the effectiveness of these vaccines in preventing viral transmission from infected to susceptible individuals has not been documented in the clinical trial reports.
In the ECDC report, the authors have thoroughly reviewed and summarized the available literature on the risk of SARS-CoV-2 transmission to susceptible individuals from reinfected COVID-19 patients previously infected with SARS-CoV-2 or immunized with a COVID-19 vaccine.
Immunity from natural infection and risk of SARS-CoV-2 transmission
A limited number of studies have been conducted to investigate the risk of viral transmission from individuals previously infected with SARS-CoV-2. According to these studies, not all individuals with confirmed SARS-CoV-2 infection develop robust, long-lasting cellular and humoral immune responses that can protect them from future reinfection. Moreover, reinfected individuals are capable of transmitting the virus to susceptible individuals.
In contrast, there is evidence indicating that reinfection is a rare event in the case of COVID-19. Several longitudinal studies involving a considerable number of participants have revealed that individuals infected with SARS-CoV-2 can achieve about 81% to 100% protection against reinfection from day 14 after the initial infection and that the protection can persist for 5 to 7 months. However, the observations made in these studies have indicated that older adults aged 65 years and above may be less protected from reinfection.
In the later phase of the pandemic, several SARS-CoV-2 variants have emerged, such as B.1.1.7, B.1.351, and P.1. Because of the presence of specific spike mutations, these variants exhibit significantly higher transmissibility than the previously circulating SARS-CoV-2 variants. Moreover, evidence suggests that these variants might be more virulent because of their ability to evade host immune responses. However, only a limited number of studies are currently available regarding the characteristics of host immune responses induced by these variants of concern. In this context, some preliminary evidence has suggested that immunity developed against the previously circulating SARS-CoV-2 variants might not be sufficient to provide robust and long-lasting protection against the newly emerged viral variants.
Taken together, these observations indicate that a significant reduction in the total number of infections is expected when a significant number of individuals will develop natural immunity against SARS-CoV-2. This, in turn, could reduce the overall risk of viral transmission.
Immunity from vaccination and risk of SARS-CoV-2 transmission
Only a single study from Scotland is available to summarize the direct effect of COVID-19 vaccination on the risk of viral transmission. According to this study, the risk of SARS-CoV-2 transmission to susceptible members of a given family can be reduced by at least 30% by vaccinating a member of the same family.
Moreover, there is evidence indicating that vaccinated individuals have significantly lower viral load and are significantly protected against symptomatic or asymptomatic infection. This, in turn, could reduce the rate of viral transmission in general. However, there might be individual - and/or vaccine product-level variations in vaccine efficacy. It is expected that the total number of infections will reduce significantly with an increase in vaccine coverage. However, the vaccine strains should be matched with the circulating viral strains in order to achieve a reduction in case numbers.
Because the COVID-19 vaccination programs have started only recently, it is not yet possible to evaluate the long-term efficacy of potential vaccines against SARS-CoV-2 reinfection. However, there is evidence indicating that anti-SARS-CoV-2 antibody levels peak at 3 – 4 weeks post-vaccination. Furthermore, only a limited number of studies have reported reduced efficacy of currently available vaccines against newly emerged SARS-CoV-2 variants.
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