SARS-CoV-2 Testing Finds Low Transmission in Rural Community

Doctor in protective gloves & workwear holding Testing Kit for the coronavirus test. The doctor is collecting nasal sample for a young lady with a sampling swab.
Investigators performed case detection of an entire rural community in Bolinas, California, to quantity SARS-CoV-2 transmission using PCR and antibody testing.

Weeks after stay-at-home orders were placed, reverse transcription polymerase chain reaction (RT-PCR) and antibody testing in a rural California community found severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection was rare, according to a study published in Open Forum Infectious Diseases.

A cross-sectional survey of infection was conducted 4 weeks after shelter-in-place orders were given in Bolinas, California where the town population is 1620. Participants were tested via PCR and 2 types of antibody testing (Abbott ARCHITECT immunoglobulin [Ig]G and in-house IgG ELISA).

Of the total 1891 participants who were tested, 1312 were residents of Bolinas, and the remainder were a mix of non-resident first responders, essential workers, volunteers, and their families. Participants were mostly adults (90%) over the age of 60 years (35%).

Of the 1847 RT-PCR tests performed, there was 0 active infections. Investigators estimated a population prevalence of 0.00048 (95% credible interval [CrI], 0.00001-0.00176). Based on an estimated test sensitivity of 80%, it would be unlikely to observe 0 infections (P <.05) if there were over 3 true infections in the town. Of the 1880 participants who received antibody tests for prior infection, 12 tested positive. Of these 12 participants, 8 were confirmed residents.

Estimated seroprevalence for Bolinas residents was 0.29% (95% CrI, 0.01-0.78%) based on the Abbott IgG test and 0.23% (95% CrI 0.01-0.62%) based on the in-house ELISA IgG test. Using data from both tests, investigators estimated a seroprevalence of 0.16% (95% CrI, 0.02-0.46%). Investigators noted that the wide range of the CrI reflects the uncertainty in interpreting a positive result in a very low prevalence setting. The positive predict value (PPV) using just a single test was 44-63% (95% CrI, 3.25-98.64%), compared to using both tests, which gives a PPV of 99.11% (95% CrI, 95.75-99.94%).

The true sensitivity of all SARS-CoV-2 antibody tests applied to community samples with a majority of mild or asymptomatic infections is currently unknown, investigators noted. Other study limitations included potential sampling bias toward subsets of the population and incomplete epidemiologic surveys by participants.

Both active and prior infections were rare in this rural town with high uptake of mask-wearing and compliance with stay at home orders, despite their proximity to urban areas experiencing significantly higher transmission, authors concluded.

Disclosure: Several study authors declared affiliations with the pharmaceutical industry. Please see the original reference for a full list of authors’ disclosures.


Appa A, Takahashi S, Rodriguez-Barraquer I, et al. Universal PCR and antibody testing demonstrate little to no transmission of SARS-CoV-2 in a rural community. Open Forum Infect Dis. Published online October 30, 2020. doi:10.1093/ofid/ofaa531