key: cord-0709169-f4d8c71f
authors: Pratò, Simone; Paladino, Maria Emilia; Riva, Michele Augusto; Deni, Matteo; Belingheri, Michael
title: SARS-CoV-2 Transmission Risk to Household and Family Contacts by Vaccinated Healthcare Workers
date: 2021-04-23
journal: J Occup Environ Med
DOI: 10.1097/jom.0000000000002238
sha: 23ff08a9fae27e9ce2e28fd65f4405dfab8c5c9c
doc_id: 709169
cord_uid: f4d8c71f

nan

To the Editor:

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).

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 ¼ 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 vaccinated. RT-PCR was performed on 46 close contacts at the beginning, 49 at the end, and 39 at both the beginning and end of the quarantine period. Negative RT-PCR results were observed in 43 of the 46 cases at the beginning of the quarantine period and in all cases (n ¼ 49) at the end of the incubation period.

Our study documented the possibility of finding asymptomatic HCWs who were RT-PCR positive after completing COVID-19 vaccination. We found that SARS-CoV-2 infection has certain characteristics in vaccinated people, such as lower viral load and shorter shedding duration, similar to a report by a preliminary observational study from Israel after the first dose of Pfizer-BioNTech vaccine. 8 In our study, asymptomatic infection with weak positivity and low viral load on RT-PCR was found in 37 of 40 patients. The British SARS-CoV-2 variant (VOC-202012/01, lineage B.1.1.7) with high viral loads were found in only three of the 40 cases. Given that the prevalence of the British variant was 89.1% in Italy by the time of this study, 9 we expected the same prevalence of this variant from our HCWs; however, The effectiveness of the COVID-19 vaccination in preventing person-to-person SARS-CoV-2 transmission remains unclear. 2,10 Crowded indoor environments with sustained close contact and conversations, such as households, are particularly high-risk settings. 11 In our study, we identified 74 close contacts composed of 61 household members and 13 other family members. The only RT-PCRs that were found positive were in three close contacts at the beginning of the quarantine period, so we do not know whether the index cases were the HCWs or their contacts. When neither the HCWs nor their contacts had symptoms, and each RT-PCR was positive at the beginning of the quarantine period, defining the index case became challenging. Therefore, the most important finding in our study was that SAR during the COVID-19 incubation period was zero, also when the quarantine in a separate living space was not possible (eg, due to the presence of very young children). This meant that nobody became positive during the incubation period, indicating that SARS-CoV-2 was not transmitted from vaccinated HCWs. Moreover, only one close contact of 74 was found symptomatic and this was at the beginning of the quarantine period.

The relationships between the contacts and HCWs were also investigated. Spouses are known to be at a higher risk than other family contacts since this may reflect intimacy, sleeping in the same room, or longer or more direct exposure to index cases. 11 We identified 23 spouses' relationships, however, RT-PCRs were consistently negative during the quarantine period, and none of them had experienced COVID-19 symptoms.

Our findings may be explained by the effectiveness of Comirnaty in preventing person-to-person SARS-CoV-2 transmission. This may be due to a reduction in viral loads, which appears to be the leading indicator of SARS-CoV-2 transmission prevention, as well as the absence of symptoms in the index cases. 11, 12 Our data are consistent with the register-based analysis from Scotland concerning the effectiveness of COVID-19 vaccination against the transmission of SARS-CoV-2, which reported a 30% risk reduction among household members of vaccinated HCWs with a single dose of either the Pfizer or AstraZeneca COVID-19 vaccine. 1 In addition, the authors reported that a 30% reduction in infection risk is likely underestimated due to the lack of information about the source of the infection, as household members of HCWs could also have been infected through other routes.

Despite these findings, our study had several limitations. The most notable is the small sample size of vaccinated HCWs with positive RT-PCR results, despite the large number of molecular tests carried out. This is likely attributable to the effectiveness of the COVID-19 vaccine, Comirnaty, in preventing infections. Another limitation concerns close contacts who did not agree to undergo molecular testing at the beginning of the quarantine or on the tenth day, preferring to undergo clinical monitoring until the end of the quarantine in some cases (eg, small children). Finally, as with other studies involving household transmission, infections from outside the home cannot always be ruled out. 

Effect of vaccination on transmission of COVID-19: an observational study in healthcare workers and their households

Risk of SARS-CoV-2 transmission from newly-infected individuals with documented previous infection or vaccination

Contributions of humoral and cellular immunity to vaccine-induced protection in humans

SARS-CoV-2 pandemia in Lombardy: the impact on family Paediatricians

Effects of COVID-19 lockdown on epidemic diseases of childhood

Prolonged PCR positivity stigma and return-to-work after SARS-CoV-2 infection

World Health Organization. Contact tracing in the context of COVID-19 -Interim Guidance

Initial report of decreased SARS-CoV-2 viral load after inoculation with the BNT162b2 vaccine

Italian National Institute of Health. Prevalence of variants in Italy: lineage B.1.1.7, P.1, P.2, lineage B.1.351, lineage B.1.525. Survey of 18

Household transmission of SARS-CoV-2: a systematic review and meta-analysis

Transmission of COVID-19 in 282 clusters in Catalonia, Spain: a cohort study