Study suggests vast majority of Seattle population remains susceptible to COVID-19

By Sally Robertson, B.Sc.Dec 9 2020

Researchers in the United States have conducted a study suggesting that a small proportion of people living in Seattle were infected with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) – the agent that causes coronavirus disease 2019 (COVID-19) – by around late March/early April 2020.

The team’s study of samples taken from adults living in the Seattle area revealed that the proportion of those positive for antibodies against SARS-CoV-2 rose from zero before March 18^th to 1.2% by late March/early April.

This is consistent with the known time frame of SARS-CoV-2 transmission in the community, says Helen Chu of the University of Washington and colleagues from Kaiser Permanente, Synergy America, Inc., Eagle Global Scientific, and the Centers for Disease Control and Prevention (CDC).

However, the low proportion of people identified as seropositive for previous SARS-CoV-2 infection suggests that most of the local community may still be susceptible to COVID-19, they add.

This has important public health implications that need to be taken into consideration as the pandemic involves, says the team.

A pre-print version of the paper is available on the medRxiv* server while the article undergoes peer review.

Study: Seroprevalence of SARS-CoV-2 Antibodies in Seattle, Washington: October 2019-April 2020. Image Credit: zstock / Shutterstock

This news article was a review of a preliminary scientific report that had not undergone peer-review at the time of publication. Since its initial publication, the scientific report has now been peer reviewed and accepted for publication in a Scientific Journal. Links to the preliminary and peer-reviewed reports are available in the Sources section at the bottom of this article. View Sources

Estimating the prevalence of SARS-CoV-2 infections is essential

“Estimating the prevalence of SARS-CoV-2 infections is essential to determining the progression of local COVID-19 pandemics,” said Chu and colleagues.

Early on in the pandemic, the majority of COVID-19 data available were from hospitalized patients, owing to the limited availability of testing in the community.

This left many public health officials with an inaccurate picture of infection incidence, including cumulative incidence and potential progression towards population or herd immunity.

“Serologic surveys for SARS-CoV-2 antibodies can help fill this knowledge gap,” says Chu and the team.

The first case of COVID-19 in United States occurred in greater Seattle

In the United States, the first case of COVID-19 was detected in the greater Seattle area of western Washington State on January 20th, 2020. Additional cases were then identified, and the first known case of a COVID-19-related death occurred on February 29th.

A state of emergency was then declared, and multiple control measures were introduced in an effort to curb the SARS-CoV-2 virus transmission.

What did the researchers do?

To evaluate the effects of these control measures and to inform future interventions, Chu and team estimated the prevalence of SARS-CoV-2 infection in the greater Seattle area between October 2019 and early April 2020.

The team obtained residual serum samples from 763 Seattle-area adults who had undergone routine screening for viral hepatitis and tested the samples for the presence of SARS-CoV-2 antibodies. The residual sera had been collected once a month from October 2019 through January 2020, and then weekly from the beginning of March 2020.

The samples were tested using a SARS-CoV-2-specific enzyme-linked immunosorbent assay (ELISA) and then stored at 4 °C before being tested again (using the same assay) to reduce the likelihood of false positives.

What did they find?

Of all the samples collected in 2019, none were confirmed positive for SARS-CoV-2.

Three confirmed positive samples were identified among samples collected on March 18^th, one was identified among samples collected on March 25^th, and one was identified among samples collected on April 1^st.

Overall, the findings suggested that the estimated seroprevalence of SARS-CoV-2 antibodies among the study cohort between March 5^th and April 1^st, 2020, was 1.2%.

“This is comparable to the seroprevalence reported in Seattle-area children around the same time period,” says the team.

Given that the epidemic may have begun earlier in Washington state than in other regions of the United States, a seroprevalence of 1.2% is low compared to cities with large epidemics such as Madrid, say the researchers. This supports the idea that the control measures taken in Washington were successful at limiting the early SARS-CoV-2 outbreak, they add.

The study “contributes important data”

Chu and colleagues say the study contributes important data to the so far limited information available on the seroprevalence of SARS-CoV-2 antibodies.

First, SARS-CoV-2 seroprevalence increased from zero before March 18^th to 1.2% in late March/early April, which corroborates the known time-frame of community transmission.

Second, the low proportion of adults in the Seattle area with serologic evidence of previous SARS-CoV-2 infection suggests that the vast majority of the local population may remain susceptible to COVID-19.

“This has important implications for public health as the pandemic evolves,” said Chu and the team.

The study has limitations and further studies are needed

The authors acknowledge that the study has several limitations that may limit the generalizability of the findings.

For example, the sample size is small, thereby limiting the accuracy of the estimates and residual samples collected from a group of patients undergoing hepatitis screening may not be representative of the Seattle population as a whole. The researchers were also unable to collect specimens in February, when SARS-CoV-2 may have been circulating.

“Future studies of population prevalence using specimens collected from a statistically representative cross-sectional cohort, over time and across diverse geographic locations, are needed to help better characterize the true incidence and public health impact of COVID-19, thereby enabling more accurate estimations of infections, and mortality rates,” advises the team.

This news article was a review of a preliminary scientific report that had not undergone peer-review at the time of publication. Since its initial publication, the scientific report has now been peer reviewed and accepted for publication in a Scientific Journal. Links to the preliminary and peer-reviewed reports are available in the Sources section at the bottom of this article. View Sources