SARS-CoV-2 Transmission Risk to Household and Family Contacts by Vaccinated Healthcare Workers

Abstract

To the Editor: Readers are invited to submit letters for publication in this department. Submit letters online at http://joem.edmgr.com. Choose “Submit New Manuscript.” A signed copyright assignment and financial disclosure form must be submitted with the letter. Form available at www.joem.org under Author and Reviewer information. A recent publication from the European Centre for Disease Prevention and Control (ECDC) on severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) transmission reported that few data are available about coronavirus disease (COVID-19) vaccination effectiveness in preventing person-to-person infection.1,2 Current COVID-19 vaccines prevent viremia, but it is still uncertain whether they provide protection against viral replication and the virus spread within the airways.3 The aim of our study was to evaluate the risk of SARS-CoV-2 transmission when new infection occurs after vaccination. Crowded indoor environments, such as domestic settings, are high-risk transmission environments for SARS-CoV-2 as contact is more likely to occur over a long period of time without personal protective equipment. Accordingly, we investigated SARS-CoV-2 transmission among a cohort of vaccinated healthcare workers (HCWs) and their household and other family contacts. Italy was one of the hardest hit areas in Europe during the pandemic in 2020. In response, the COVID-19 vaccination campaign was launched on December 27, 2020, with the first available vaccine, Pfizer/BioNTech's Comirnaty. In order to detect new cases of COVID-19 among vaccinated subjects, nose swab tests with real-time polymerase chain reaction (RT-PCR) were performed on asymptomatic, vaccinated HCWs of a hospital in northern Italy from February to March 2021.4,5 METHODS From February to March 2021, 1196 asymptomatic HCWs from a northern Italian hospital were routinely tested with RT-PCR for SARS-CoV-2 every 2 weeks. All workers were vaccinated with two doses of Comirnaty based on the technical data sheet, and HCWs with a previous COVID-19 infection were not tested within 90 days after diagnosis in accordance with international guidelines.6 Furthermore, SARS-CoV-2 genotypes from nasal swabs (as used in RT-PCR virus testing) were analyzed to investigate the effectiveness of the vaccine against virus variants. Each HCW who was RT-PCR positive at least 7 days after the second vaccine dose was interviewed by a physician to gather information about the characteristics of his/her close contacts (occupation, vaccination status, symptoms, RT-PCR tests, comorbidities). The interviews were conducted using a questionnaire provided by the Italian National Institute of Health for COVID-19 contact tracing in the case of infections. We enrolled these close contacts within 48 hours before the first RT-PCR positive result or during the 10-day quarantine period, focusing mostly on household members and other close family contacts given the greater risk of transmission among these categories.7 All close contacts were asked to be tested with RT-PCR at the beginning and end of the quarantine period. To rule out infections from outside the home (eg, non-household contacts) and to avoid household transmission overestimation, we ruled out contacts at higher risk of exposure to SARS-CoV-2 than the general population due to other possible sources of infection (eg, close contacts at higher risk of exposure due to their occupations). On the other hand, we also ruled out vaccinated close contacts because they are at a lower risk for infection. From the selected close contacts, they were classified as either household members or as other family contacts (eg, those due to car trips). A household member is defined as any person living in the same house, and the other family contacts were relatives of the index case who met him/her within 1 m in an enclosed space for more than 15 minutes without personal protective equipment.7 For each category, the secondary attack rate (SAR) was calculated by dividing the number of positive contacts by the total number of close contacts (ie, the proportion of contacts that tested positive). RESULTS From February to March 2021, 40 of 1196 asymptomatic HCWs were RT-PCR positive at least 7 days after the second vaccine dose. Their characteristics are shown in Table 1. The median age was 35 years (range: 27 to 69 years), and the majority were women (n = 31). In addition, there were seven HCWs who already had a previous COVID-19 infection. During the quarantine period with a median duration of 10 days (range: 10 to 25 days), 37 of the 40 HCWs remained asymptomatic. Of the 40 cases, 33 HCWs were RT-PCR negative after 10 days. From there, we identified 74 close contacts of the 40 vaccinated HCWs who tested positive for RT-PCR (Table 1). A total of eight close contacts were excluded; four were excluded because of a higher risk of exposure to SARS-CoV-2 due to their jobs, and the other four were excluded because they had already been vacci