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Abstract

Background

Despite the unprecedented measures implemented globally in early 2020 to prevent the spread of SARS-CoV-2, Sweden, as many other countries, experienced a severe first wave during the COVID-19 pandemic.

Aim

We investigated the introduction and spread of SARS-CoV-2 into Sweden.

Methods

We analysed stored respiratory specimens (n = 1,979), sampled 7 February–2 April 2020, by PCR for SARS-CoV-2 and sequenced PCR-positive specimens. Sequences generated from newly detected cases and stored positive specimens February–June 2020 (n = 954) were combined with sequences (Sweden: n = 730; other countries: n = 129,913) retrieved from other sources for Nextstrain clade assignment and phylogenetic analyses.

Results

Twelve previously unrecognised SARS-CoV-2 cases were identified: the earliest was sampled on 3 March, 1 week before recognised community transmission. We showed an early influx of clades 20A and 20B from Italy (201/328, 61% of cases exposed abroad) and clades 19A and 20C from Austria (61/328, 19%). Clade 20C dominated the first wave (20C: 908/1,684, 54%; 20B: 438/1,684, 26%; 20A: 263/1,684, 16%), and 800 of 1,684 (48%) Swedish sequences formed a country-specific 20C cluster defined by a spike mutation (G24368T). At the regional level, the proportion of clade 20C sequences correlated with an earlier weighted mean date of COVID-19 deaths.

Conclusion

Community transmission in Sweden started when mitigation efforts still focused on preventing influx. This created a transmission advantage for clade 20C, likely introduced from ongoing cryptic spread in Austria. Therefore, pandemic preparedness should have a comprehensive approach, including capacity for large-scale diagnostics to allow early detection of travel-related cases and community transmission.

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2024-10-10
2024-12-12
http://instance.metastore.ingenta.com/content/10.2807/1560-7917.ES.2024.29.41.2400021
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Early unrecognised SARS-CoV-2 introductions shaped the first pandemic wave, Sweden, 2020
<p class="citation"> <span>Citation style for this article:</span> <span class="meta-value authors"> <a href="/search?value1=Robert+Dyrdak&option1=author&noRedirect=true" class="nonDisambigAuthorLink">Dyrdak Robert</a>, <a href="/search?value1=Emma+B+Hodcroft&option1=author&noRedirect=true" class="nonDisambigAuthorLink">Hodcroft Emma B</a>, <a href="/search?value1=Sandra+Broddesson&option1=author&noRedirect=true" class="nonDisambigAuthorLink">Broddesson Sandra</a>, <a href="/search?value1=Malin+Grabbe&option1=author&noRedirect=true" class="nonDisambigAuthorLink">Grabbe Malin</a>, <a href="/search?value1=Hildur+Franklin&option1=author&noRedirect=true" class="nonDisambigAuthorLink">Franklin Hildur</a>, <a href="/search?value1=Magnus+Gissl%C3%A9n&option1=author&noRedirect=true" class="nonDisambigAuthorLink">Gisslén Magnus</a>, <a href="/search?value1=Maricris+E+Holm&option1=author&noRedirect=true" class="nonDisambigAuthorLink">Holm Maricris E</a>, <a href="/search?value1=Magnus+Lindh&option1=author&noRedirect=true" class="nonDisambigAuthorLink">Lindh Magnus</a>, <a href="/search?value1=Joanna+Nederby-%C3%96hd&option1=author&noRedirect=true" class="nonDisambigAuthorLink">Nederby-Öhd Joanna</a>, <a href="/search?value1=Johan+Ringlander&option1=author&noRedirect=true" class="nonDisambigAuthorLink">Ringlander Johan</a>, <a href="/search?value1=Martin+Sundqvist&option1=author&noRedirect=true" class="nonDisambigAuthorLink">Sundqvist Martin</a>, <a href="/search?value1=Richard+A+Neher&option1=author&noRedirect=true" class="nonDisambigAuthorLink">Neher Richard A</a>, <a href="/search?value1=Jan+Albert&option1=author&noRedirect=true" class="nonDisambigAuthorLink">Albert Jan</a></span>. Early unrecognised SARS-CoV-2 introductions shaped the first pandemic wave, Sweden, 2020. <a href="/content/ecdc">Euro Surveill.</a> 2024;29(41):pii=2400021. <a href="https://doi.org/10.2807/1560-7917.ES.2024.29.41.2400021" title="doi link" target="_blank">https://doi.org/10.2807/1560-7917.ES.2024.29.41.2400021</a> <span class="ES_Article_citation"> <span class="generated">Received</span>: 03 Jan 2024;&nbsp;&nbsp; <span class="generated">Accepted</span>: 30 May 2024 </span> </p>
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