Immunoglobulin G Responses to SARS-Cov-2 and Patterns of Adverse Vaccination Effects Among Health Care Workers in North-Eastern Tanzania: A Retrospective Longitudinal Study

Abstract

Background: The global response to the COVID-19 has been largely successful due to widespread vaccination
programs, which have resulted in significant clinical and socioeconomic achievements. Nonetheless, there is a significant
information gap on the efficacy and safety of COVID-19 vaccines. Scientific data on basic immunological attributes of
COVID-19 vaccines such as; duration of protection, and potential side effects associated with vaccination is inadequate,
leading to high hesitancy rates towards vaccination. This study aimed at bridging these knowledge gaps and addressing
these crucial issues.
Methods: This was a retrospective longitudinal study involving 273 health care workers (HCWs) from Kilimanjaro
Christian Medical Centre, a referral zonal hospital in northern Tanzania, between August 2022 and February 2023.
Immunoglobulin G concentrations were measured over a 21 months period post- COVID-19 vaccination using an indirect
Enzyme Linked Immunosorbent Assay (ELISA). Data was analysed using STATA software version 15 (College Station, TX).
Descriptive statistics was used to summarise the study participant’s characteristics and prevalence of antibodies against
SARS-CoV-2. Kruskal-Wallis and Mann–Whitney U test were used to assess the differences between exposure variables
and median SARS COV-2 IgG concentration. Logistic regression was used to determine association of independent
variables with seroprevalence, using a p-value of .05 as the cut off for statistical significance.
Results: The study population of HCWs at the KCMC is strongly seropositive to COVID-19. Vaccinated individuals had
a significantly higher median IgG concentration (137.5 IU/ml) than unvaccinated individuals (122.12 IU/ml) (p<.01).
Individuals who received a booster vaccination dose showed a higher median IgG concentration (145.7 IU/ml)
compared to those who received a single dose (137.5 IU/ml). Our findings identified two IgG concentration peaks at
5 months (136.17U/ml) and 17 months (146.4 IU/ml) post vaccination. These peaks align with the peaks in immune
response following vaccination and natural exposure during the second COVID-19 wave, respectively. Regarding
adverse effects, only a few HCWs reported side effects after vaccination, and these were not found to be associated
with any specific host factors.
Conclusions: Majority of HCWs at KCMC were seropositive to COVID-19 during the study period. The combination of
vaccination and natural exposure to SARS-CoV-2 contributed to the high seropositivity rate among HCWs in the study
site. Vaccine related adverse effects were rare among recipients indicating a high degree of safety of the vaccines.
Further studies are warranted to better understand and characterise immune responses in terms of longevity and the level
of protection conferred by vaccination and natural exposure.