Coronavirus disease 2019 (COVID-19), caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), is a worldwide public health concern for healthcare workers, including physicians, public health specialists and researchers.¹ The World Health Organization declared the outbreak of COVID-19, which was first reported in Wuhan, China, as a Public Health Emergency of International Concern on 30 January 2020,² as it was posing a high risk to countries with vulnerable health systems.³ Almost all 55 countries in Africa have been affected by the coronavirus pandemic, with sub-Saharan Africa being the most affected.⁴ Zambia recorded its first case of COVID-19 on 18 March 2020⁵ and has since recorded a total of 333 555 COVID-19 cases and 4017 deaths as of 04 October 2022.⁶

Severe acute respiratory syndrome coronavirus causes a number of human respiratory disease conditions, ranging from mild cold to severe respiratory distress syndrome, and it largely spreads between persons by respiratory droplets and contact routes.⁷ On the other hand, SARS-CoV-2 has been seen to spread faster than SARS-CoV, which was first reported in 2003 and caused previous outbreaks. Accumulating evidence showed that SARS-CoV-2, unlike SARS-CoV, is transmitted by persons without symptoms.⁸ About 80% of cases appear to be asymptomatic,^4,9,10 and about 3.3% may experience hospitalisation and later die. Based on global biological, epidemiological and modelling evidence, asymptomatic COVID-19 may play a substantial role in the pandemic trajectory.¹¹

Despite public health preventive measures such as hand hygiene, social distancing, quarantine and travel restrictions which were instituted, transmission of COVID-19 seemed to be ongoing.¹² Vaccination against COVID-19 has been shown to be effective against contracting severe forms of the disease.¹³ However, vaccination does not protect an individual from transmitting or becoming infected with SARS-CoV-2.¹⁴

Zambia has experienced four waves of COVID-19.¹ During the first wave to about the fourth wave of COVID-19, it was a requirement to have a negative polymerase chain reaction COVID-19 certificate by everyone intending to travel.^15,16 This study aimed to assess the positivity rate among asymptomatic travellers during the second wave of the COVID-19 pandemic, and its determinants.

A total of 11 144 asymptomatic travellers tested for COVID-19 were sampled for the study and 1781 tested positive, resulting in a positivity rate of 16.0% (Table 1). The study participants had a median age of 36 years (interquartile range: 29 to 36 years). The test for COVID-19 was noted to be done mostly by travellers in the age group 19 to 50 years. The youngest participant was 1 year old while the oldest was 92 years old. A majority of those tested were male travellers (73.2%; 8152/11 144); female travellers accounted for the remaining 26.8% (2992/11 144). The highest number of tests were completed in January 2021 (4170/11 144).

The proportion of female travellers that tested positive for COVID-19 (18.4%) was greater than the proportion of male travellers (15.1%) with a p-value of 0.027 (Table 2). Among individuals who tested for COVID-19 prior to travelling, about 7303 (83.2%) tested negative and were Ndola residents, whereas among individuals that tested positive, 304 (12.9%) were not Ndola residents. There were no positive results among individuals whose samples were collected orally. Of the monthly tests done, 30/1557 (1.9%) were positive in December 2020, 1060/4170 (25.4%) were positive in January 2021, 532/3079 (17.3%) positive in February 2021 and 159/2338 (6.8%) positive in March 2021.

When stratified by month of visit to the testing centre, there were more Ndola residents seeking testing services at the TDRC laboratory in January 2021 (n = 948) than any other month included in the study (Table 3). An equal peak number of non-Ndola residents (n = 111) was seen in January 2021 and February 2021. The lowest number of travellers was seen in December 2020.

Coronavirus disease 2019 cases began to rise on 05 January 2021 and reached a peak on 26 January 2021 (Figure 1). The cases remained high until 23 February 2021, when there was a reduction of 150 in the number of positive cases reported.

Female travellers had a 16.0% (adjusted odds ratio: 1.16; 95% confidence interval: 1.03 – 1.30; p = 0.012) increased chance of testing positive for SARS-CoV-2 compared to male travellers, after adjusting for age, residence, and month in which the test was done (Table 4).

This study found a positivity rate of 16.0% (1781/11 144), with 69.1% (1231/1781) of male travellers being affected. The months of January 2021 and February 2021 recorded the highest rate of positivity. The epidemiological curve showed that the second wave of COVID-19 lasted from December 2020 to the end of March 2021. Further, the chances of testing positive for SARS-CoV-2 if an individual was female increased by about 16% (95% confidence interval: 1.03–1.30) compared to being male, after controlling for other variables.

The positivity rate found in this study was similar to the positivity rate at the national level in Zambia during the same period.⁶ Despite the similarity, the national level positivity rate comprised both symptomatic and asymptomatic cases. The positivity rate found could have been higher if control measures of isolation and quarantine of cases and testing of people before travel were not put in place and followed.¹¹ On the other hand, the positivity rate in this study was higher than the national rate of 10.6% during the first wave.¹⁵ This could have been due to differences in the attack rate and rate of transmission of the strain of coronavirus.¹⁶ Conversely, a study in Nigeria reported a higher positivity rate of 20.8% in the second wave which lasted from 25 October 2020 to 03 April 2021, with asymptomatic cases being the majority.¹⁷ A study done by Avadhanula et al. showed a positivity rate of 11.4% among asymptomatic patients during the second wave between 18 March 2020 and 15 August 2020 in Houston, Texas, United States, which was much lower than the rate found in this study.¹⁸ This could have been due to differences in region, rate of transmission, adherence to recommended guidelines and utilisation of COVID-19 vaccine as it was introduced in some countries earlier than others.¹⁷ Our study and a study by Ghosh, Sarkar and Chouhan, done in India between March 2021 and May 2021, thus confirmed the presence of COVID-19 among asymptomatic individuals.¹⁹

Our study found that a rise in COVID-19 cases during the second wave of the pandemic was observed from December 2020 and ended in March 2021. The epidemiological curve for the daily cases of COVID-19 showed a peak on 26 January 2021. In Italy, different findings were reported in which the second wave began in August 2020 and continued to February 2021.²⁰ A study in Spain demonstrated that the second wave started on 01 July 2020 and ended on 15 October 2020, indicating that this period was different to what was obtained in our study.²¹ In India, the peak of cases was observed around 01 March 2021.¹⁹ These differences could be a result of differences in geographical locations and climatic conditions across the globe.^22,23,24

This study found a statistically significant difference in positivity rate between female travellers compared with male travellers, with female travellers more likely to test positive. These findings are consistent with those in Nigeria, where more asymptomatic female individuals than male tested positive.¹⁷ Other studies have also reported similar findings in which female individuals had higher odds of positivity than male individuals.^25,26 This could be due to female patients having a higher health-seeking behaviour than male patients.²⁵ In a study done in Netherlands, on data collected from March 2020 to August 2020, there was no significant difference in positivity rates between female patients and male patients.²⁷ The study also found the age between 19 to 50 years to have a higher positivity rate compared to those who were 18 years or younger and those older than 50 years. This finding was not different from the study done in Wuhan, China, and Bahrain, Ireland, that reported a higher prevalence of COVID-19 in individuals who were less than 45 years old in Wuhan, and 20 to 49 years in Bahrain.^28,29 This could be because those aged 18 years and younger were less susceptible to COVID-19 during the second wave.³⁰ In addition, control measures such as closure of school, colleges and universities may have contributed to the age group 18 years and younger having a low positivity rate.³¹ On the other hand, most of the individuals older than 50 years were symptomatic and prone to hospitalisation as compared to younger individuals.³²