What Is the Antibody Response and Role in Conferring Natural Immunity After SARS-CoV-2 Infection? Rapid, Living Practice Points From the American College of Physicians (Version 1)

Antibody tests for SARS-CoV-2, the virus that causes COVID-19, are widely available. These practice points help clinicians, patients, and public health professionals understand how to use and interpret these results on the basis of the available evidence. The practice points will be updated as more evidence becomes available.

Abstract

Description: The widespread availability of SARS-CoV-2 antibody tests raises important questions for clinicians, patients, and public health professionals related to the appropriate use and interpretation of these tests. The Scientific Medical Policy Committee (SMPC) of the American College of Physicians developed these rapid, living practice points to summarize the current and best available evidence on the antibody response to SARS-CoV-2 infection, antibody durability after initial infection with SARS-CoV-2, and antibody protection against reinfection with SARS-CoV-2.

Methods: The SMPC developed these rapid, living practice points based on a rapid and living systematic evidence review done by the Portland VA Research Foundation and funded by the Agency for Healthcare Research and Quality. Ongoing literature surveillance is planned through December 2021. When new studies are identified and a full update of the evidence review is published, the SMPC will assess the new evidence and any effect on the practice points.

Practice Points: Practice Point 1: Do not use SARS-CoV-2 antibody tests for the diagnosis of SARS-CoV-2 infection.

Practice Point 2: Antibody tests can be useful for the purpose of estimating community prevalence of SARS-CoV-2 infection.

Practice Point 3: Current evidence is uncertain to predict presence, level, or durability of natural immunity conferred by SARS-CoV-2 antibodies against reinfection (after SARS-CoV-2 infection).

Key Question 1

What are the prevalence, level, and durability of detectable anti–SARS-CoV-2 antibodies among patients infected with or recovered from reverse transcriptase polymerase chain reaction (RT-PCR)–diagnosed SARS-CoV-2 infection?

Key Question 1a

Do the levels and durability of detectable antibodies vary by patient characteristics (for example, age, sex, race/ethnicity, and comorbidities), COVID-19 severity, presence of symptoms, time from symptom onset, or the characteristics of the immunoassay (sensitivity or specificity)?

Key Question 2

Do anti–SARS-CoV-2 antibodies confer natural immunity against reinfection?

Key Question 2a

Does natural immunity vary by such factors as initial antibody levels, patient characteristics, presence of symptoms, or severity of disease?

Key Question 2b

Is there a threshold level of detectable anti–SARS-CoV-2 antibodies necessary to confer natural immunity, and if so, does this threshold vary by patient characteristics (for example, age, sex, race/ethnicity, and comorbidities)?

Key Question 3

If anti–SARS-CoV-2 antibodies confer natural immunity against reinfection, how long does this immunity last?

Key Question 3a

Does the duration of natural immunity vary by such factors as initial antibody levels, patient characteristics, presence of symptoms, or severity of disease?

Key Question 4

What are the unintended consequences of antibody testing after SARS-CoV-2 infection?

Background

The widespread availability of SARS-CoV-2 antibody tests raises important questions for clinicians, patients, and public health professionals related to the appropriate use and interpretation of these tests. However, currently little is known about the relationship between SARS-CoV-2 antibodies and natural immunity. The potential for natural immunity to SARS-CoV-2 infection stems from the activation of B lymphocytes (humoral or antibody-mediated immunity) and T lymphocytes (cellular immunity). However, like with other viruses, the relationship between antibodies and natural immunity may vary on the basis of differences in the level and duration of antibodies produced as well as viral mutations of the infection. When persons are infected with SARS-CoV-2, uncertainty exists about whether the antibodies produced (IgM, IgG, IgA, or neutralizing) are protective against reinfection, and if so, for how long what levels of antibodies are needed for such protection (1). In addition, because antibodies to other coronaviruses have been shown to decline over time, how long such protection against reinfection may last also needs to be determined (2). As a step toward better understanding the immune response to SARS-CoV-2, the Scientific Medical Policy Committee (SMPC) of the American College of Physicians (ACP) developed these practice points on the basis of key questions related to the antibody-mediated natural immunity after SARS-CoV-2 infection. This article does not evaluate cellular immunity or artificial immunity conferred by vaccines, both of which are important areas of research.

The SMPC developed these rapid, living practice points (Table 1) on the basis of a rapid and living systematic evidence review done by the Portland VA Research Foundation and funded by the Agency for Healthcare Research and Quality (3, 4). The details of our process can be found in the Appendix. This version of the practice points is based on an initial search to 4 August 2020 that was subsequently revised and updated through 15 December 2020. It was approved by ACP's Executive Committee of the Board of Regents on behalf of the Board of Regents on 22 February 2021 and submitted to Annals of Internal Medicine on 22 February 2021. Ongoing literature surveillance is planned through December 2021. The target audience for these practice points includes clinicians, patients, the public, and public health officials. The target patient population includes adults who have been previously infected with SARS-CoV-2.

Table 1. Practice Points.

Table 2 presents clinical considerations, the Figure and Table 3 summarize current evidence, and Table 4 identifies additional evidence gaps. The Appendix Table presents the data estimates supporting the practice points.

Table 2. Clinical Considerations.

Figure. Evidence description.

Evidence search and assessment done by the Portland VA Research Foundation (3, 4). Updated search for evidence updated through 15 December 2020.

* Observational studies include studies estimating seroprevalence among a given population that includes a small subpopulation known to have SARS-CoV-2 and cross-sectional or cohort studies characterizing the antibody response among adults with SARS-CoV-2 infection. Immunoassay validation studies include those validating the diagnostic performance of 1 or more immunoassays (3, 4).

Table 3. Evidence Summary for Patients With PCR-Confirmed SARS-CoV-2 Infection.

Table 4. Evidence Gaps.

Appendix Table. Data Estimates.

Practice Points and Rationale

Prevalence, Level, and Durability of Antibodies Among Patients Infected With or Recovered From SARS-CoV-2 Infection

Practice Point 1: Do not use SARS-CoV-2 antibody tests for the diagnosis of SARS-CoV-2 infection.

Practice Point 2: Antibody tests can be useful for the purpose of estimating community prevalence of SARS-CoV-2 infection.

Studies included in the evidence review focused on evaluating the trends in types of antibodies and their levels after symptom onset or confirmation of SARS-CoV-2 infection with a positive RT-PCR test result. Evidence from studies evaluating community prevalence in antibody response showed that patients develop an immune response after SARS-CoV-2 infection. This is evidenced by detectable IgA antibodies in most patients (low certainty), IgM in most patients (moderate certainty), IgG in nearly all patients (moderate certainty), and neutralizing antibodies in nearly all patients (low certainty). The antibody prevalence and levels may vary over time by certain patient characteristics (for example, age, sex, and race/ethnicity) and disease factors (for example, presence of symptoms and severity) (low certainty). The timing from symptom onset or PCR-confirmed infection of when antibodies first become detectable and the level at which they remain detectable vary depending on the type of antibody. At or around peak level, IgM, IgG, IgA, and neutralizing antibodies are estimated to be detectable in approximately 80%, 95%, 83%, and 99% of patients, respectively, after symptom onset or PCR-confirmed infection. Despite variation, each of these antibody types has its peak level on average between 20 and 31 days after symptom onset or PCR-confirmed infection. Evidence shows that antibodies may persist over time; IgM antibodies were detected up to 115 days (moderate certainty), IgG antibodies were detected up to 120 days (moderate certainty), IgA antibodies were detected up to 140 days (low certainty), and neutralizing antibodies were detected up to 152 days (low certainty).

Given that not all patients develop detectable antibodies early in the course of the infection and that the presence and levels may vary by patient and disease characteristics, antibody tests should not be used for the diagnosis of SARS-CoV-2 infection. It is also important for clinicians and patients to keep in mind that SARS-CoV-2 antibody test results may be falsely positive due to cross-reactivity with antibodies of other coronaviruses (74, 75). Furthermore, although a complete assessment of diagnostic accuracy of various antibody tests was beyond the scope of the evidence review, characteristics (for example, sensitivity, specificity, and accuracy) varied substantially among the antibody tests used in included studies (3, 4). Such variation can contribute to false-negative and false-positive test results and ultimately wrong conclusions (76, 77).

However, for the purposes of estimating community prevalence of SARS-CoV-2 infection, antibody testing is a feasible option, keeping in mind that antibody levels peak roughly 3 to 5 weeks after symptom onset or PCR diagnosis. Also, the usability and interpretation of SARS-CoV-2 antibodies will need to be evaluated in persons vaccinated against COVID-19, as vaccination will also affect the development of SARS-CoV-2 antibodies.

Reinfection Among Patients With SARS-CoV-2 Antibodies and Unintended Consequences of Antibody Testing

Practice Point 3: Current evidence is uncertain to predict presence, level, or durability of natural immunity conferred by SARS-CoV-2 antibodies against reinfection (after SARS-CoV-2 infection).

Current evidence is limited about natural immunity conferred by SARS-CoV-2 antibodies. As discussed earlier, asymptomatic or symptomatic patients may develop an antibody response consistent with natural immunity after having SARS-CoV-2 infection, but key individual-level differences depend on such variables as COVID-19 disease severity, patient factors, types of antibodies and amount developed, and how long the antibodies last. This is an area of rapidly emerging new evidence. No identified evidence directly evaluates the association between antibodies and natural immunity, although 2 studies are in progress (7, 78). In the evidence review, a study (8) of hospitalized patients with COVID-19 (n = 47) reported a potential case of reinfection during the “convalescence stage” of the disease in 1 patient who did not have detectable IgM or IgG antibodies at 4-week follow-up. However, the study was not designed to determine whether antibodies confer immunity. Evidence does show that there are detectable levels of IgA antibodies in most patients (low certainty), IgM in most patients (moderate certainty), IgG in nearly all patients (moderate certainty), and neutralizing antibodies in nearly all patients (low certainty). Evidence also shows that IgG antibodies probably remain detectable for at least 120 days (moderate certainty) and neutralizing antibodies may remain detectable for at least 152 days (low certainty). The antibody prevalence and levels over time may vary by certain patient characteristics (for example, age, sex, and race/ethnicity) and disease factors (for example, presence of symptoms and severity) (low certainty). The evidence review also identified 3 longitudinal studies (indirect evidence) that used serologic rather than RT-PCR testing as the index test and, thus, did not meet the inclusion criteria. These studies suggest that antibody presence may be associated with natural immunity (78–81); however, the evidence review has not critically appraised them. Given that there is no direct evidence to inform the question of reinfection, we will consider modifying future searches to formally incorporate additional sources of indirect evidence, including these studies.

Evidence is uncertain (insufficient) about the unintended consequences of antibody testing.

Given limited knowledge about the association between antibody levels and natural immunity, patients with SARS-CoV-2 infection and those with a history of SARS-CoV-2 infection should follow recommended infection prevention and control procedures to slow and reduce the transmission of SARS-CoV-2 (5, 6).

Appendix: Practice Points Development Process

The SMPC, in collaboration with staff from ACP's Department of Clinical Policy, developed these practice points on the basis of a rapid and living systematic evidence review done by the Portland VA Research Foundation and funded by the Agency for Healthcare Research and Quality (3, 4). The SMPC comprises 11 internal medicine physicians representing various clinical areas of expertise and 1 public (nonclinician) member and includes members with expertise in epidemiology, evidence synthesis, health policy, and guideline development. In addition to contributing clinical, scientific, and methodological expertise, Clinical Policy staff provided administrative support and liaised among the SMPC, the evidence review funding entity and evidence team, and the journal. Clinical Policy staff and the SMPC reviewed and prioritized potential topic suggestions from ACP members, SMPC members, and ACP governance. A committee subgroup, including the SMPC chair, worked with staff to draft the key questions and led the development of the practice points. Clinical Policy staff worked with the subgroup and an independent evidence review team to refine the key questions and determine appropriate evidence synthesis methods for each key question. Via conference calls and e-mail, Clinical Policy staff worked with the committee subgroup to draft the practice points on the basis of the results of the rapid and living systematic evidence review. The full SMPC reviewed and approved the final practice points. Before journal submission, ACP's Executive Committee of the Board of Regents also reviewed and approved the practice points on behalf of the ACP Board of Regents. The evidence review team is planning ongoing literature surveillance at least through December 2021. When no new studies are identified, the SMPC will publish a comment on the most recent version of the practice points that indicates the date of the last search and that no new studies were identified. When new studies are identified but previous conclusions remain unchanged, the SMPC will publish an update alert letter that briefly summarizes the new evidence and updates the rationale and evidence tables for the practice points. When new studies are identified and a full update of the evidence review is published, the SMPC will assess the new evidence and reaffirm (via update alert letter) or revise and modify (via new version) the practice points. The SMPC will continually evaluate the priority level of each living topic and may decide to retire a topic early from living status if it determines that the topic is no longer considered a priority for decision making, if there is confidence that the conclusions are not likely to change with the emergence of new evidence or affect practice, or when it is unlikely that new evidence will emerge (82).