British Columbia’s successful response to COVID-19 – policy anchored by science
By Sally Robertson, B.Sc.Jul 16 2020
Researchers at the University of British Columbia and the BC Center for Disease Control in Vancouver have reported the first estimates of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) seroprevalence for Canada before and after mitigation measures were relaxed in May.
Danuta Skowronski and colleagues say the findings support other data indicating that community transmission was successfully suppressed throughout the winter-spring period in British Columbia.
“Less than 1% of British Columbians had been infected with SARS-CoV-2 when first-wave mitigation measures were relaxed in May 2020,” writes the team.
However, this success “constitutes a double-edged sword,” say the authors since the seroprevalence monitoring also showed significant residual susceptibility to the virus.
The researchers say their monitoring protocol represents a reliable, convenient, and efficient approach to estimating population seroprevalence and can be easily applied for further monitoring as the pandemic continues to unfold.
A pre-print version of the paper is available in the server medRxiv*, while the article undergoes peer review.
June 17,2020 Vancouver British Columbia Canada Yellow umbrellas in Yaletown will cover the square as a commitment to flattening the curve the fight against COVID-19. Image Credit: FPfotografy / Shutterstock
British Columbia has been recognized for its control of SARS-CoV-2
In Canada, the first reported cases of COVID-19 imported from China were in Vancouver on January 15th and in Ontario on January 25th.
British Columbia (BC) also reported a super-spreading event on March 6th and an outbreak in a care facility on March 9th.
Since then, BC has been recognized for its control of SARS-CoV-2 during the winter/spring period this year, with surveillance data demonstrating that incidence and death rates in the province were amongst the lowest in Canada. This success had been attributed to the timely warning, rapid deployment of testing, and limitations on staff movement between care facilities.
As in other Canadian provinces, BC advised against non-essential travel on March 12th and introduced various intervention measures, including bans on public gatherings and closure of personal service establishments. These restrictions lasted approximately two months, with the relaxation of the measures beginning by May 19th.
Despite other provinces having implemented similar interventions, COVID-19 incidence and death rates in BC consistently remained amongst the lowest in Canada. They also remained amongst the lowest compared with adjacent west coast U.S. states and the rest of the world.
Statistics led to speculation
However, up to early April, testing in BC was restricted to high-risk or exposed individuals, giving a per capita testing rate that was lower than in other provinces. Furthermore, BC did not differ to anywhere else in terms of its all-cause death rate between March 15th and April 25th, exceeding that of the previous 5 years.
“Taken together, these statistics have led to speculation that BC may have experienced more undocumented SARS-CoV-2 infections that, in conjunction with asymptomatic cases, contributed to unrecognized community transmission,” write the authors.
Monitoring seroprevalence at the beginning and end of the first wave
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Sofronski and team performed serial cross-sectional sampling at the beginning (March 5th – 13th) and end (May 15th – 27th) of the first pandemic wave using anonymized residual sera obtained from labs in areas of BC where community attack rates were expected to be highest.
Two high-throughput, chemiluminescent immuno-assays were performed, one targeting the S1 subunit of the SARS-CoV-2 spike protein (that the virus uses bind host cells) and one targeting the viral nucleocapsid.
Samples that were seropositive on either of the first two assays were screened by a third assay for the S1 receptor-binding domain (RBD) and assessed for neutralizing antibodies.
Analysis of the March and May “snapshots” generated an estimated community-wide SARS-CoV-2 infection rate of less than 1% throughout the winter-spring period.
The results for March
Basing seroprevalence on dual-assay positivity, the team estimated a cumulative incidence of 0.28% by the start of the first wave in March.
However, only two of 869 samples collected were seropositive against both S1 and S1-RBD, and none were seropositive against both S1 and nucleocapsid. Furthermore, none had detectable neutralizing antibodies.
“In that regard, the only two dually-positive specimens in March may have been false-positives, and community-level attack rates could be even lower than we estimate by that snapshot,” writes the team.
On the other hand, people with asymptomatic or mild infections may not have mounted an antibody response or had sustained titers of neutralizing antibodies, they add.
The results for May
By time restrictions were eased in May, the estimated seroprevalence and cumulative incidence were still low, at 0.55%.
Unlike the March snapshot, four of 885 samples showed S1 and nucleocapsid positivity, as well as being seropositive against S1-RBD and having detectable neutralizing antibodies.
However, the team says that if they had applied this 0.55% estimate to the Lower Mainland source population, which is approximately 3 million, the number of infections would have been around 16,500, which is about eight times higher than the number recorded.
“A double-edged sword”
Skowronski and colleagues say their seroprevalence findings reinforce other surveillance data indicating SARS-CoV-2 transmission was successfully suppressed throughout the winter-spring period in BC.
“This success, however, constitutes a double-edged sword, further highlighting substantial residual susceptibility,” they add. “Our seroprevalence protocol is readily amenable to comparison across serial snapshots, and these are planned at relevant intervals as the pandemic unfolds.
medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.