There are concerns about the effectiveness of COVID-19 vaccines against immune escape by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants of concern (VOCs). Delta is currently the most common VOC worldwide, and nasal swabs suggest that fully vaccinated people harbor infectious viral loads just as unvaccinated persons do.
Most studies of the prevalence of vaccine breakthrough infection (VBT) among health care workers (HCWs) have relied on spontaneous self-reporting by symptomatic HCWs, thus ignoring the possibility of asymptomatic individuals infecting other individuals.
In most countries, individuals with a positive COVID-19 test are instructed to undergo a 14-day quarantine, and the protocol for HCWs recommends two consecutive follow-ups after at least ten days before reinstating workplace visits. However, many of these countries are also experiencing massive HCW shortages due to the current health crises and in-patient surges.
A new study published in the journal Annals of Internal Medicine conducted systematic surveillance involving 2,397 HCWs who were fully vaccinated with the Pfizer-BioNTech BNT162 vaccine at the ASST Settelaghi hospitals, Italy, in February 2021.
Here, fully-vaccinated HCWs who worked in eight hospitals wards and were deemed to be at “high risk” were tested via polymerase chain reaction (PCR) every two weeks, while those in eight “moderate-risk” wards were tested every four weeks. If SARS-CoV-2 RNA was detected, the HCW was tested daily until two consecutive tests were negative. Additionally, positive samples with cycle thresholds (Ct) below 25 were retested by spike gene sequencing. While positive samples with Ct in the range of 25-35 were retested with a VOC-specific multiplex real-time PCR.
Overall, 33 SARS-CoV-2 positive cases were identified between 15 March and 17 August 2021. Among these, 17 belonged to the high-risk group, and 16 were in the moderate-risk group. Of noteworthy importance, none of these HCWs developed symptoms nor had immune deficiency or significant comorbid conditions.
Five cases were characterized by spike gene sequencing and 12 by VOC-specific real-time PCR. Eight HCWs (0.4%) tested positive during the study period while symptomatic; only 1 variant could be characterized as Alpha by sequencing.
Meanwhile, all 33 asymptomatic patients tested negative the day after the initial positive result, and all negative results were confirmed on a third nasopharyngeal swab test, two days later. Whereas symptomatic cases had negative test results after a mean of 11 days. The mean IgG level was twice as high in the asymptomatic group.
The findings suggested that the incidence of asymptomatic VBT infections varied with the frequency of testing. In fact, with the same testing frequency, the incidence was found to be similar in the high- and moderate-risk groups.
Asymptomatic cases cleared faster and had higher mean antibody levels than symptomatic cases. Therefore, the surveillance program could have missed many other asymptomatic VBTs. This is the first study depicting symptomatic and asymptomatic cases among HCWs during the Delta predominance.
It was inferred that regular testing of asymptomatic vaccinated HCWs can provide an accurate picture of VBTs. Therefore, more extended quarantines for symptomatic patients who test positive for SARS-CoV-2 may be necessary. Moreover, surveillance programs that aim to record all VBT infections would need to include frequent testing. Furthermore, strict adherence to preventive measures, such as the use of personal protective equipment, is warranted.