Distribution of SARS-CoV-2 outbreaks in Thailand
From March 2020 to May 2021, 113 confirmed cases (1st wave; N = 8, 2nd wave; N = 40 and state quarantine; N = 65) were successfully genotyped by partial SARS-CoV-2 genome sequencing. In this study, the first wave of the outbreak (March–May 2020) in Thailand was characterised by two different lineages, A and B.1 (Figs. 1 and S1). During the period of SQ from May 2020 through May 2021, we received 65 clinical specimens obtained from travellers and Thais who presented with/without symptoms of COVID-19 and were admitted to a hospital/hotel in Bangkok and Chon Buri province. Among these, lineage B.1.1 was the most frequently detected genotype and accounted for 29.2% of the strains (19/65), followed by 48% (31/65) for lineage B.1. Of the remaining strains, 11 were classified as lineage B.1.1.7 (Alpha) and another three of lineage B.1.177 (Figure S1 and Table S2). All of these strains from SQ were imported from the Americas (12.3%), Asia (41.5%), Europe (23.1%), and unknown (23.1%). The results showed that lineage B.1.1.7 (Alpha) was imported from the United States, France, Slovenia, the United Kingdom (UK), and Northern Ireland. Lineage B.1.177 was imported from the UK and the United Arab Emirates (UAE). Lineage B.1.1 was predominantly imported from Asian countries (Qatar, India, Philippines, Japan, and Bahrain), the UK, and Italy. Lineage B.1 was imported from Asian and European countries. During the second wave of the outbreak (October 2020–March 2021), lineage B.1 became the predominant virus.
Multiplex real-time RT-PCR assays to differentiate variants of SARS-CoV-2
In Thailand, predominant variants were detected in different epidemic waves (Fig. 2). From March 2020 to 14 March 2022, 5,627 samples were identified as B.1.1.7 (Alpha), B.1.617.2 (Delta), and B.1.1.529 (Omicron BA.1 and BA.2) from SARS-CoV2 positive samples by using multiplex real-time RT-PCR. The results showed that clade B.1.1.7 (Alpha) was the most frequent variant in the third epidemic wave (1,510/5,627: 26.8%). B.1.617.2 (Delta) (2,382/5,627: 42.3%) was the predominant strain responsible for SARS-CoV-2 infection in the fourth epidemic wave. B.1.1.529 (omicron BA.1) was first detected in Thailand in mid-December 2021 (1,375/5,627: 24.4%), which caused a nationwide epidemic wave until the end of February 2022. Since then, clade B.1.1.529 (omicron BA.2) emerged in Thailand at the end of January 2022 (360/5,627: 6.4%) and became the major variant in early March 2022.
Phylogenetic relationship
The complete spike nucleotide sequences of the 155 SARS-CoV-2 strains were investigated to evaluate the genetic relationship among Thai SARS-CoV-2 strains. A total of 155 positive strains consisting of clade B.1.1.7 (Alpha) (N = 20), B.1.617.2 (Delta) (N = 33), B.1.1.529 (omicron BA.1) (N = 48), and B.1.1.529 (omicron BA.2) (N = 54), were characterised by amplification of the partial S gene, which harbours the major antigenic sites in SARS-CoVs. Sequences determined in isolates from 113 samples during the first and second epidemic waves and SQ were also included. The phylogenetic relationships defined with the S sequences showed that clade B.1.1.529 (omicron BA.1 and BA.2) cluster together and were differentiated from other clades with bootstrap values > 95%.
The S gene analysis showed that the B.1.1.7 (Alpha) differed from the Wuhan-Hu1 strain by 7 amino acid substitutions: N501Y, A570D, D614G, P681H, T716I, S982A and D1118H. The B.1.177 variants contained L16F, D215H, A222V, N370S and D614G mutations. Several mutations were identified in the sequences of the B.1.617.2 (Delta) variants, namely, T17R, T93I, G140D, L452R, T478K, D614G, P681R, and D950N. Forty-two and 31 amino acid substitutions were detected in the B.1.1.529 (omicron BA.1) and the B.1.1.529 (omicron BA.2), respectively. In addition, one strain (Thailand_CU8056) carried an I1221T substitution in the S protein that was clustered in the B.1.1.529 (omicron BA.2) clade (Fig. 3).
The evolutionary history of the structural region of SARS-CoV-2 was investigated by performing Bayesian analysis with a SARS-CoV-2 S glycoprotein sequence data set. The mean evolution rate was 2.60×10− 3 (95% highest posterior density [HPD], 1.72×10− 4 to 3.62×10− 4) substitutions per site year (Fig. 4). The most recent common ancestor of all SARS-CoV-2 clades was dated in September 2019.