Trends in SARS-CoV-2 seroprevalence among pregnant women attending first antenatal care visits in Zambia: A repeated cross-sectional survey, 2021-2022

Abstract

SARS-CoV-2 serosurveys help estimate the extent of transmission and guide the allocation of COVID-19 vaccines. We measured SARS-CoV-2 seroprevalence among women attending ANC clinics to assess exposure trends over time in Zambia. We conducted repeated cross-sectional SARS-CoV-2 seroprevalence surveys among pregnant women aged 15-49 years attending their first ANC visits in four districts of Zambia (two urban and two rural) during September 2021-September 2022. Serologic testing was done using a multiplex bead assay which detects IgG antibodies to the nucleocapsid protein and the spike protein receptor-binding domain (RBD). We calculated monthly SARS-CoV-2 seroprevalence by district. We also categorized seropositive results as infection alone, infection and vaccination, or vaccination alone based on anti-RBD and anti-nucleocapsid test results and self-reported COVID-19 vaccination status (vaccinated was having received ≥1 dose). Among 8,304 participants, 5,296 (63.8%) were cumulatively seropositive for SARS-CoV-2 antibodies from September 2021 through September 2022. SARS-CoV-2 seroprevalence primarily increased from September 2021 to September 2022 in three districts (Lusaka: 61.8-100.0%, Chongwe: 39.6-94.7%, Chipata: 56.5-95.0%), but in Chadiza, seroprevalence increased from 27.8% in September 2021 to 77.2% in April 2022 before gradually dropping to 56.6% in July 2022. Among 5,906 participants with a valid COVID-19 vaccination status, infection alone accounted for antibody responses in 77.7% (4,590) of participants. Most women attending ANC had evidence of prior SARS-CoV-2 infection and most SARS-CoV-2 seropositivity was infection-induced. Capturing COVID-19 vaccination status and using a multiplex bead assay with anti-nucleocapsid and anti-RBD targets facilitated distinguishing infection-induced versus vaccine-induced antibody responses during a period of increasing COVID-19 vaccine coverage in Zambia. Declining seroprevalence in Chadiza may indicate waning antibodies and a need for booster vaccines. ANC clinics have a potential role in ongoing SARS-CoV-2 serosurveillance and can continue to provide insights into SARS-CoV-2 antibody dynamics to inform near real-time public health responses.

Fig 1. Reported COVID-19 cases among the general population and SARS-CoV-2 seroprevalence among study participants, by district.

The bars represent COVID-19 case among all persons in the district reported by Zambia National Public Health Institute and the lines represent the monthly SARS-CoV-2 seroprevalence among pregnant women attending antenatal care in the district during the study. Serologic testing was done using the Tetracore FlexImmArray SARS-CoV-2 Human IgG Antibody Test; a positive result was assigned if signal ratios ≥1.2 for all 3 assay targets (nucleocapsid [N], receptor-binding domain [RBD] of the spike protein, and N-RBD fusion). Seroprevalence was adjusted for assay performance (sensitivity = 89.8% and specificity = 100%) and error bars represent 95% confidence intervals.

Fig 2. Antibody response categorization based on anti-RBD IgG, anti-nucleocapsid IgG, and COVID-19 vaccination status (N = 8,304).

The Tetracore FlexImmArray SARS-CoV-2 Human IgG Antibody Test used is a multiplex bead assay with three SARS-CoV-2 targets (i.e., nucleocapsid, receptor-binding domain (RBD) of spike, and fusion) to facilitate distinguishing between infection only, vaccination only, and combined vaccination and infection antibody responses when considered with vaccination history. Participants who reported receiving a Sinopharm or unknown vaccine were categorized as indeterminate due to the inability to distinguish between vaccination only and hybrid vaccination and infection antibody responses from inactivated or attenuated virus vaccines. Adapted from Duarte et al. 2021.

Fig 3. SARS-CoV-2 antibody response categorization of study participants by enrollment month and district.

No evident antibody response was defined as testing negative for anti-RBD IgG. Infection-derived antibody response was defined as positive anti-RBD IgG AND no reported COVID-19 vaccination. Vaccination and infection-derived hybrid antibody response was defined as positive anti-RBD IgG AND positive anti-nucleocapsid IgG AND reported COVID-19 vaccination. Vaccination-derived antibody response was defined as positive anti-RBD IgG AND reported COVID-19 vaccination AND negative anti-nucleocapsid IgG. Equivocal IgG responses, unknown COVID-19 vaccination status, and unknown or Sinopharm vaccine type were categorized as indeterminate.

Fig 4. Signal ratios for three assay targets by antibody response category.

Signal ratios were calculated by dividing the mean fluorescent intensity of the target by the mean fluorescent intensity of the calibrators. Vaccination and infection combined produced the strongest antibody response against all three assay targets compared to infection or vaccination alone. Participants whose antibody response was derived from infection only had greater signal ratios for the RBD target compared to participants with vaccination-derived antibody response only. ns = not significant (p-value>0.05); **p-value<0.001.