Coronavirus disease 2019 (COVID-19), was confirmed to be caused by SARS-CoV2, a novel coronavirus, by the Chinese authorities on 7 January 2020, following a cluster of patients with pneumonia of unknown aetiology linked to the seafood market in Wuhan City, Hubei Province of China in December 2019 [1]. Due to the rapid spread across the globe, the declaration of a COVID-19 pandemic was made in March 2020 by World Health Organization (WHO).

Transmission of SARS-CoV2 in Southeast Asian countries was not as widespread in 2020 as in other parts of the world despite close proximity to China [2]. This was attributed to early border closure at the end of January and early February 2020 [3]. In Laos, where public health infrastructure is limited, issuance of visas to travelers from China was suspended starting on February 2, 2020 [4], followed by restriction on all international travel beginning on March 29, 2020 [5]. Other non-pharmaceutical interventions, such as mask-wearing, were implemented in early February 2020, followed by school and workplace closures, as well as the prohibition of mass gatherings, in March 2020 [5]. Lao PDR was not only the last country in the region to report its first confirmed COVID-19 cases on March 24, 2020 [6], but it also had the lowest number of COVID-19 cases compared to neighbouring countries such as Cambodia, Thailand, Vietnam, and Myanmar [3]. Following the first confirmed case, the number of confirmed cases remained low until the end of April 2021, after which it gradually increased and surged at the end of August 2021 [5]. Mass vaccination was first administered to frontline workers and healthcare workers in March 2021, followed by the general public in April 2021 using Sinopharm COVID-19 vaccine from China and AstraZeneca from the COVAX facility [7,8].

In Laos, the reported number of COVID-19 cases was primarily based on testing symptomatic individuals, which may have overlooked asymptomatic cases. Estimates of asymptomatic infections from other countries varied from less than one percent to over 50% [9]. He et al. found that asymptomatic rate could be up to 80% in Wuhan, China [10]. Underestimation of the true number of infections can also result from availability, accessibility, sensitivity and specificity of diagnostic tests in use. In Laos, RT-PCR was initially only available in the capital city, Vientiane. By September 2020, ten provincial laboratories had the capacity to test for COVID-19 using GeneXpert machines with plans to expand testing to the remaining provinces thereafter [11].

Although viral RNA detection by real-time RT-PCR is the most reliable technique to confirm infections or incidences, it detects active or current infection only. In contrast, antibody tests can indicate past exposure to the infection, even in asymptomatic individuals who might have been missed by PCR testing or those who recovered a long time ago. Therefore, a better estimation of the prevalence of SARS-CoV-2 infections can be achieved using serological tests, which help to understand the distribution and level of SARS-CoV-2 infection [12].

The first extensive seroprevalence study in Laos, conducted between August and September 2020, included community members and healthcare workers across five provinces, including Vientiane Capital, as well as individuals with wildlife contacts in Vientiane province. This study found no evidence of significant SARS-CoV-2 circulation [13].

To provide more data to estimate the nationwide prevalence of SARS-CoV2 in Laos before and at the early stage of the pandemic, we conducted a seroprevalence study of SARS-CoV2 in blood donors from each province across the country by detecting anti-SARS-CoV-2 neutralizing antibodies (NAb) in stored blood donor samples collected during the pre-pandemic and pandemic periods.

Laos was considered vulnerable to COVID-19 due to its shared border with China, where SARS-CoV-2 was first reported. However, similar to other ASEAN countries, the number of reported cases was relatively low throughout 2020 and into 2021 [2]. A systematic review and meta-analysis on the global seroprevalence of SARS-CoV-2 antibodies in 2020 showed that seroprevalence in the general population was low, with a median of 4.5% (IQR: 2.4–8.4%), and could be as high as 59% among persons working in care facilities. The study also found that Southeast Asia had the lowest seroprevalence at 0.6%, compared to 19.5% in Sub-Saharan Africa [15]. Another repeated study, which collected data between January 2020 and May 2022, showed that by September 2021, the global SARS-CoV-2 seroprevalence increased to 59.2% (95% CI: 56.1%–62.2%). The study also reported a steep increase in seroprevalence in Africa, rising from 26.6% in early 2021 to 88.5% by the end of 2021 [16]. From the first reported case in March 2020 up to the first two months of our prospective sampling (February-March 2021), only 49 confirmed cases were reported [17]. Our study confirmed low level of community transmission of COVID-19 throughout Laos before May 2021 with only two out of 2251(0.09%) samples collected testing positive. Subsequently, there was noticeable rise in the seroprevalence among blood donors, with 218 out of 1,412 (15.4%) samples testing positive. This increase corresponded with the rise in reported cases, from 49 cases in March 2021–555 cases by the end of April and 15,095 cases by the end of August 2021 [5,18]. Our findings are in accordance with the first seroprevalence survey conducted by Virachith and colleagues between August and September 2020 in community and high-risk groups in Vientiane capital, Luang-Prabang, Oudomxay, Savannakhet and Champasak provinces, confirming that the community transmission was low until September 2020 [13]. They found that 5.2% (131/2433) and 2.1% (19/1061) of the general population tested positive for anti-N antibodies and anti-S antibodies tested by ELISA, respectively; 2% (13/666) and 1.4% (4/282) of healthcare workers; and 20.3% (15/74) and 6.8% (5/74) of individuals with bat/wildlife contact. However, only 0.1% (2/3173) of participants were positive for both antibodies and the authors believe that the seropositivity observed in their study was due to cross-reactivity with non-SARS-CoV-2 coronaviruses.

We found the highest prevalence in three northern provinces (Bokeo, Luang-Namtha and Oudomxay) and one southern province (Salavan) of Laos, but all of the samples positive for anti-SARS-CoV-2 antibodies were collected after April 2021, when COVID-19 vaccines had already been rolled out and the number of clinical cases was beginning to increase. During the study period, the highest number of reported COVID-19 cases, according to the government, was in Savannakhet, followed by Champasack, Khammuan, and Vientiane Capital [5]. Unfortunately, the number of blood donors sampled from Khammuan, Savannakhet, Phongsaly, and Sekong was limited, preventing us from obtaining SARS-CoV-2 seroprevalence data for these provinces. The low number of positive samples in Vientiane Capital and Champasack Province in our study may be attributed to the fact that most blood donations were collected before May, a period of low transmission.

There was speculation that the actual case numbers might have been underestimated, potentially due to low testing rates or to pre-existing immunity generated by exposure to other related coronavirus circulating in the area prior to the pandemic [19]. However, analysis of 676 samples collected between 2003 and 2018 revealed no detection of SARS-CoV-2 neutralizing antibodies.

The COVID-19 situation report at the end of May 2021 indicated that Huaphan, Sekong, Attapeu, and Xaisomboun were still in Stage 0, with no reported cases in these four provinces [5]. However, in Attapeu, one out of 78 blood donor samples collected in February and eight out of 64 samples collected in May tested positive for antibodies against SARS-CoV-2. Additionally, three out of 22 blood donor samples collected in May in Houaphanh were positive, suggesting some silent circulation of SARS-CoV-2 in these two provinces despite the absence of confirmed cases.

Using blood donors for a seroprevalence study was very convenient for identifying infection hotspots. However, there are several limitations to using blood donors, such as selection bias, age and gender distribution, and geographical bias. In our study, the most significant limitation was the timing of blood donations, as the samples collected were not equally distributed over the study period in each province. For example, our study was conducted prospectively between February and August 2021; some provinces sent nearly all their samples collected before May 2021, while others sent samples collected after that, which distort the actual epidemiology of the infection.

Community transmission of SARS-CoV-2 in Laos before May 2021 was low all over the country, which was consistent with government reports. This was likely due to strict control measures implemented. The high prevalence in certain provinces also aligned with the reported increase in cases and the rollout of the COVID-19 vaccine.