The SARS-CoV-2 Delta variant might cause high viral loads, is highly transmissible, and contains mutations that confer partial immune escape 1,2. Outbreak investigations suggest that vaccinated persons can spread Delta 3,4. We compared RT-PCR cycle threshold (Ct) data from 699 swab specimens collected in Wisconsin 29 June through 31 July 2021 and tested with a qualitative assay by a single contract laboratory. Specimens came from residents of 36 counties, most in southern and southeastern Wisconsin, and 81% of cases were not associated with an outbreak. During this time, estimated prevalence of Delta variants in Wisconsin increased from 69% to over 95%. Vaccination status was determined via self-reporting and state immunization records (Supplemental Figure 1).
We observed low Ct values (<25) in 212 of 310 fully vaccinated (68%; Figure 1A) and 246 of 389 (63%) unvaccinated individuals. Testing a subset of low-Ct samples revealed infectious SARS-CoV-2 in 15 of 17 specimens (88%) from unvaccinated individuals and 37 of 39 (95%) from vaccinated people (Figure 1B). Low Ct values were detected in vaccinated people regardless of symptoms at the time of testing (Figure 1C). Ct values <25 were detected in 7 of 24 unvaccinated (29%; CI: 13-51%) and 9 of 11 fully vaccinated asymptomatic individuals (82%; CI: 48-97%), and 158 of 232 unvaccinated (68%, CI: 62- 74%) and 156 of 225 fully vaccinated (69%; CI: 63-75%) symptomatic individuals. Time from symptom onset to testing did not vary by vaccination status (p=0.40; Supplemental Figure 2). Infectious virus was detected in the sole specimen tested from an asymptomatic fully vaccinated individual. Although few asymptomatic individuals were sampled, these results indicate that even asymptomatic, fully vaccinated people might shed infectious virus. Combined with other studies 2–5 , these data indicate that vaccinated and unvaccinated individuals infected with the Delta variant might transmit infection. Importantly, we show that infectious SARS-CoV-2 is frequently found even in vaccinated persons when specimen Ct values are low. The inclusion ofviruses from Pango lineages B.1.617.2, AY.2, and AY.3, and multiple counties without a linkingoutbreak, indicate that Delta-lineage SARS-CoV-2 can achieve low Ct values consistent with
transmissibility in fully vaccinated individuals across a range of settings. Vaccinated and unvaccinated
persons should get tested when symptomatic or after close contact with someone with suspected or
confirmed COVID-19. Continued adherence to non-pharmaceutical interventions during periods of high
community transmission to mitigate spread of COVID-19 remain important for both vaccinated and