In our study, 77% of the participants reported having experienced an adverse event after receiving either dose of the vaccine. The most commonly experienced adverse events were injection site events, headache, and fever. Most adverse events had an onset within 3 days of receiving the vaccine and most events lasted between 1–3 days. In addition to that, those aged 20–29 years, 30–39 years and 40–49 years were more likely to develop adverse events than those aged ≥ 50 years. No serious adverse events were reported.
Most (77%) participants in our study revealed that they experienced an adverse event after either dose of the vaccine. As in most vaccines, an immune response is induced after AstraZeneca vaccination (18–20). These immune responses can lead to adverse events and these events occur in different persons differently (20, 21). In a prospective single-cohort study in Ethiopia among health workers to assess for adverse events after receiving the Oxford-AstraZeneca vaccine, 68% of the participants reported an adverse event (22). Despite a difference in the study designs, the results were similar. Similarly, in Togo, 72% of participants reported at least one adverse event after vaccination with AstraZeneca vaccine (ChAdOx1 nCoV-19 vaccine). However, in an online cross-sectional study assessing for self-reported adverse events in Bangladesh, 51% of the participants reported having experienced an adverse event after receiving a dose of the Oxford-AstraZeneca (Covishield) vaccine (23). This proportion might have been lower due to the nature of the study. In an online interview, a participant may not be able to freely express their symptoms. On the contrary, in a more engaging mode of communication such as phone interviews used in this study, participants may be led on to remember or recall some of the adverse events experienced. Furthermore, in an online interview, the participants may respond hurriedly thereby missing out on some adverse events experienced. We also compared proportions of individuals who experienced adverse events with other vaccines. An online cohort study in the United States analyzing with participant-reported adverse events after COVID-19 vaccination revealed that 65% reported an adverse effect after receiving Pfizer–BioNTech COVID-19 vaccine (BNT162b2) and 80% after Johnson % Johnson vaccine (24). A large-scale community-based study in the United Kingdom reported an even much lower rate of 33.7% (25). The variation may be due to the difference in study design and the heterogeneity of the populations. Some evidence has pointed towards ethnic differences and vulnerability to adverse events following vaccination (26).
We found that the most reported adverse events included injection site events, fever, headaches, and general body weakness. This is because the vaccine instructs the body immune system to react in certain ways including increases in blood flow more at the injection site, so more immune cells can circulate, and it raises the body temperature. This is consistent with other studies that reported similar reported adverse events among the population (24, 27–29). In most cases, adverse events are expected and people receiving the vaccine need to be sensitized. This helps to provide assurance and prepare them psychologically. Furthermore, health workers were also trained in managing adverse events as most are expected.
No serious adverse events were reported in our study. We believe this is due to short-lived, self-limiting symptoms that are mild or moderate in severity. Most adverse events were self-resolved with none requiring inpatient hospitalization, resulting in death, a permanent/persistent disability, or a congenital anomaly. A study in Ethiopia on adverse events among health care workers who received the Oxford/AstraZeneca vaccine noted similar results to our study. (22). Several studies have also reported no serious adverse events following vaccination with AstraZeneca (25, 30). In China, a meta-analysis of 12 different vaccines at phase 3 level of clinical trials revealed that the odds of serious adverse events following at least one dose of mRNA vaccines (AOR: 1.47; 95% CI: 0.65–3.3) was higher compared to those who received at least one dose of non-replicating viral vector vaccines (AOR: 0.76; 95% CI: 0.62–0.93) and inactivated vaccines(AOR: 0.79, 95% CI: 0.62–1.00) (31). However, the same study revealed that no solid evidence indicated that COVID-19 vaccines directly caused serious adverse events.
In our study, individuals aged 20–29 years, 30–39 years and 40–49 years were more likely to develop adverse events than those aged ≥ 50 years. Furthermore, the odds increased with decreasing age compared to those aged 50 years and above. This may be due to higher reactogenicity among younger people than their older. Younger individuals tend to have a more active immune system which wanes with increasing age (32). Similarly, several studies have noted increasing odds of adverse events among younger individuals following administration of AstraZeneca COVID-19 vaccine and other COVID-19 vaccines (22, 28, 30, 33). A comparative study among 3 different vaccines noted similar results (34). However, older individuals might report less adverse events as they often dismiss them as symptoms of old age. This has been shown in a non-COVID-19 related study (35). This implies that adverse events among elder individuals should be well-probed. Furthermore, older individuals and their caretakers where necessary should be sensitized to always seek medical advice where adverse events present.
Limitations of the study
Our study had some limitations. First, phone interviews were voluntarily reported by individuals; thus, they might not have been objective. Secondly, the data collected might also have been subjected to recall bias as a few weeks had passed for some participants before they were subjected to the questionnaire. This was addressed through proper explanation of the study purpose and detailed probing without leading the participants onto what to say during the interview. We also called back participants where we sought clarity was necessary. Use of a prospective study would alleviate this study limitation.