Early Postnatal Infection of Neonates Born to Mothers Infected by SARS-CoV-2 Omicron Variant

This prospective, multicenter study was conducted at 10 COVID-19 maternity centers located in Lombardy, Northern Italy, between December 15, 2021 and March 16, 2022. The study protocol was approved by the ethics committee of the promoting center (Comitato Etico Milano Area B–Fondazione IRCCS Ca’ Granda Ospedale Maggiore Policlinico, Milano) and by the local committees of participating centers. Participating centers collected written informed consent from parents before inclusion in the study. All procedures were performed in accordance with the Declaration of Helsinki.²⁵  At participating centers, all mothers were screened for SARS-CoV-2 positivity through nasopharyngeal swabs before hospital admission for delivery, and then in case of symptoms suggestive for COVID-19 during hospitalization. Mothers who tested positive at delivery or within the first 2 days after childbirth before hospital discharge and considered eligible for rooming-in were consecutively enrolled if they provided consent for study participation. During the hospital stay, rooming-in was performed in isolated COVID-19 postnatal wards, with mother-infant dyads housed in single or double rooms. Dyads were cared for by dedicated staff donning full protective equipment before entering the ward. Before starting rooming-in, all mothers were instructed to wear a face mask (preferably filtering facepiece 2) during breastfeeding or when taking care of their infant, frequently apply alcohol-based solutions, and maintain physical distancing (2 meters or more) from their infant when not wearing a face mask. Neonates born to SARS-CoV-2-positive mothers were tested for SARS-CoV-2 RNA by nasopharyngeal swab 3 times at the following timepoints, according to regional guidelines²⁶ : within 24 hours of life, between 24 and 72 hours of life, and on the day of hospital discharge. Neonates who tested positive at birth were reassessed within 24 hours to confirm or exclude a possible in utero infection. Neonates of mothers diagnosed as infected in the 2 days after delivery were tested at maternal diagnosis, 24 to 72 hours later, and at discharge. After discharge, parents were invited to attend a follow-up program consisting of clinical evaluations and neonatal SARS-CoV-2 testing by nasopharyngeal swab scheduled at 10 to 15 and 20 to 25 days of life, followed by additional visits and testing in case of persisting positivity for SARS-CoV-2. Intrauterine neonatal infection was defined only in case of positive neonatal nasopharyngeal swab within 24 hours of life confirmed by a second positive swab within the following 24 hours. Infection during rooming-in was defined in the case of a positive neonatal swab between 24 hours of life and hospital discharge, and in the presence of a previous negative swab before 24 hours of life. New infection during follow-up was defined in case of 1 or more positive neonatal swabs after hospital discharge, with previous swabs negative for SARS-CoV-2.

All neonates enrolled in this study were born to mothers infected by SARS-CoV-2 at delivery or becoming infected in the first 2 days after childbirth and eligible to practice rooming-in. Term and preterm definitions followed the World Health Organization classification.²⁷  Neonates were considered eligible for rooming-in if they had a gestational age ≥34 weeks, a birth weight ≥2000 grams, and a normal physical examination with no concern about respiratory distress, feeding difficulty, or other disease. Mothers were considered eligible for rooming-in if they did not need respiratory support or oxygen supplementation and were in sufficient clinical conditions to take care of their neonates. Maternal and neonatal data were collected in a pseudonymized electronic database. Maternal data recorded were twin pregnancy, mode of delivery, vaccination status, time of SARS-CoV-2 infection diagnosis, COVID-19-related symptoms, and personal protective equipment used during rooming-in. Neonatal data recorded were sex, gestational age, birth weight, twins, Apgar score at 1 and 5 minutes of life, need for resuscitation at birth (at least ventilation with mask), skin-to-skin practice, type of feeding, need for temporary separation from the mother during rooming-in practice, discharge date, timing and results of SARS-CoV-2 nasopharyngeal swabs, any symptoms attributable to SARS-CoV-2 infection developed during hospitalization or during follow-up period, and death. Regional SARS-CoV-2 sequencing data, were collected from the Italian COVID-19 Genomic (I-Co-Gen) platform by the Italian National Institutes of Health and from the Johns Hopkins University CSSE COVID-19 data published online at OurWorldInData.org (retrieved from: https://ourworldindata.org/coronavirus#explore-the-global-situation), respectively.

Maternal and neonatal nasopharyngeal samples were collected by means of a sterile, flocked nylon swab (Copan Italia S.p.A.) and treated as previously described.¹⁷  RNA was extracted from a universal transport medium, and real-time reverse transcriptase-polymerase chain reaction targeting RNA-dependent RNA polymerase, N, ORF1, and/or E genes (depending on the specific centers’ protocols) were used to detect the presence of SARS-CoV-2 according to the World Health Organization guidelines.⁴ 

Results are presented as descriptive analysis. All categorical variables were summarized as frequencies and percentages, whereas continuous variables were summarized as mean and standard deviation or median and interquartile range (IQR) according to their distribution. GraphPad Prism version 9.0.1 (GraphPad Software, LLC) was used to analyze and represent data.

A total of 497 neonates were born to 479 mothers with SARS-CoV-2 infection at delivery or in the first 2 days after delivery at the participating centers over a period of 14 weeks, between mid-December 2021 and mid-March 2022. Among them, 35 neonates were born before 34 weeks’ gestational age, 3 had major congenital malformations, 29 were never roomed in during their hospital stay, and the parents of 9 neonates did not consent to participate. Moreover, 53 neonates were born during the transition period between a delta and omicron variant, ie, before Jan 5, 2022, when the delta variant still represented >10% of sequenced isolates in the region (Fig 1 and Fig 2). Those neonates were excluded from further analysis. Therefore, 368 neonates (2 pair of twins, 179 males) born to 366 mothers were enrolled in the current study.

Demographic and clinical characteristics of mothers and neonates are provided in Table 1. One-half of the women (194, 53%) were vaccinated against SARS-CoV-2 with at least 1 dose, whereas 146 women (39.9%) were not vaccinated at all. The vaccination status was not known for 26 (7.1%) women. A total of 303 women (82.8%) were asymptomatic at diagnosis, 38 (10.4%) experienced mild symptoms, 21 (5.7%) had moderate symptoms, and none got the disease in its severe form. The number of vaginal deliveries was 274 (74.9%), and skin-to-skin was practiced for 262 (71.2%) neonates. Most babies were born full-term, with a median gestational age of 39 weeks (IQR, 38–40), a mean (standard deviation) birth weight of 3288 (429.5) grams, and a median Apgar score of 9 to 10 at 1 to 5 minutes of life, respectively. Only 10 (2.7%) neonates needed resuscitation at birth. No infant died during the study period.

All the mother-infant dyads in this study performed rooming in during their hospital stays, either exclusively or temporarily (Table 2). Exclusive rooming-in was practiced by 319 out of 368 (86.7%) dyads; in 11 (3%) cases rooming-in was started right after delivery but was subsequently interrupted (either for maternal or neonatal clinical needs), whereas in 38 (10.3%) cases, an initial separation was necessary before rooming-in could be started. Because maternal education has been proven to greatly limit mother-to-infant postnatal transmission of SARS-CoV-2,¹⁷  all mothers were instructed to comply with the preventive measures. Breastfeeding was always encouraged during the study period; 233 (63.3%) neonates were exclusively breastfed, 107 (29.1%) breastfed neonates were supplemented with formula, and 23 (6.3%) babies were fed exclusively formula milk.

Overall, 44 out of 368 neonates (12%) tested positive for SARS-CoV-2 on nasopharyngeal swabs, either during hospital stay or after discharge (Table 3). The neonates with a positive swab result at birth not confirmed within the first 24 hours from the first positivity were not diagnosed as intrauterine infected. A possible intrauterine transmission, according to WHO definitions,⁹  was detected in 1 of 365 cases (0.3%, 3 neonates not tested within 24 hours of life), whereas the postnatal mother-to-infant transmission rate was 12.1%. Among the postnatally infected neonates, 19 (6.2%) had a first positive swab during rooming-in (Fig 3A), and 18 had a first positive swab during the follow-up (6.6% of the 271 infants with available follow-up data, corresponding to 48.6% of all neonates positive after birth). Postnatal infections at home occurred with higher frequency concurrently with the peak of COVID-19 cases in Italy (Fig 3B). Among all SARS-CoV-2-positive infants, 97.7% were always asymptomatic, whereas only 1 (2.3%) experienced mild symptoms (rhinitis) during follow-up evaluations.

Since the beginning of the COVID-19 pandemic in early 2020, the focus of researchers in perinatal medicine has been mainly focused on the probability and biological bases of in utero mother-to-infant transmission of SARS-CoV-2 from infected pregnant women. More recent evidence suggests that the risk of in utero transmission is limited, between 1% and 5.7%.^5,28  This variability can be explained by the different definitions of vertical transmission applied,^6,9  the neonatal biospecimens sampled, and potentially, the viral variant involved, although no data concerning the vertical transmission rate of B.1.1.529 (omicron) variant and subvariants were available yet.

SARS-CoV-2 could also be transmitted from infected mothers to neonates after birth like other respiratory viruses, through droplet and airborne routes. We previously reported a postnatal transmission rate of SARS-CoV-2 of 1.6% during the first pandemic wave in Italy,¹⁷  which was dominated by D614G variants.²⁹  In our setting, this low rate of transmission was favored by preventive recommendations given to mothers, such as wearing face masks and applying alcohol-based solutions when taking care of their neonates. Since their appearance in late 2021, all omicron subvariants of SARS-CoV-2 have been characterized by increased transmissibility compared with previously circulating variants, such as alpha and delta.^30,31  Therefore, we reevaluated the rate of neonatal infection in neonates born to SARS-CoV-2-positive mothers during the first month of life, in the same area, in a time span of virtually exclusive omicron circulation, and with the same recommendations provided to infected mothers.

After 2 years of the pandemic, most women enrolled in our cohort were asymptomatic, none of them presented with severe COVID-19, and ∼50% had received at least 1 dose of the anti-SARS-CoV-2 vaccine. These data confirm the robust scientific evidence revealing the reduced severity of COVID-19 after the delta era, possibly due to a combination of intrinsic lower pathogenicity of SARS-CoV-2 omicron variants and the diffusion of preexisting immunity, either conferred by previous infection or vaccination.^31–33  In Lombardy, Italy, the delta variant was rapidly replaced by omicron from late December 2021 to early January 2022, as confirmed by sequencing data; therefore, despite the fact that we did not sequence all maternal nor neonatal samples, it can be reasonably assumed that almost all women in our cohort were infected by the omicron variant after early January 2022.

Robust scientific data reveal a minimal risk of SARS-CoV-2 transmission to breastfed infants through breast milk,^5,14,34,35  and breastfeeding in case of maternal COVID-19 is now firmly recommended by several scientific organizations worldwide.^13,36  In the current study, >92% of neonates enrolled were breastfed, at least partially, at hospital discharge. This rate, similar or slightly higher to that reported in other cohorts of SARS-CoV-2-positive mothers,^37,38  reflects the rate recorded in the same area on a similar population of neonates born to SARS-CoV-2-positive mothers in 2020 (95%),¹⁷  confirming that over >2 years of the pandemic, the practice has not changed; attention given to breastfeeding by both health care providers and mothers has been consistently high.

In our cohort, 12% of all neonates tested positive for SARS-CoV-2 within the first 20 days of life. Excluding 1 case of positivity within the first 24 hours of life (confirmed possible vertical transmission according to the WHO definition) and neonates without available follow-up data, the postnatal transmission rate of SARS-CoV-2 during the virtually exclusive circulation of the omicron variant was 12.1%. To our knowledge, this is the first report on the postnatal mother-to-infant transmission of SARS-CoV-2 during an era of predominant omicron variant and subvariant circulation. This infection rate, almost equally divided between the hospital stay and home discharge, is significantly higher than the rate recorded in 2020 in a similar setting¹⁷  and occurred despite the consistent education on preventive measures provided to infected mothers. Considering the virtual absence of infective SARS-CoV-2 in human breast milk^34,35  that justifies the above-mentioned support for breastfeeding, it can be reasonably assumed that positive neonates were infected through droplets or airborne transmission. Therefore, the high transmissibility of omicron subvariants^23,30,31  is confirmed even in the context of early postnatal transmission. In participating centers, parental visits were not allowed in the postpartum COVID-19 wards, health care providers always donned full personal protective equipment when entering the COVID-19 maternity ward, and women could be hosted in single or double rooms. Therefore, at least theoretically, neonates could have acquired the infection from their mothers, both before and after hospital discharge, or from positive roommates during hospital stays. Moreover, transmission of the virus from other households or contacts after hospital discharge cannot be excluded, whereas transmission from infected unrecognized health care providers, although reported in other contexts,³⁹  seems unlikely because of the strict control precautions applied. Interestingly, postdischarge neonatal infections occurred only after mid-January 2022 when the omicron variant was almost exclusive in the Lombardy region, and concurrently with the peak of positive samples recorded in Italy, with no cases after February 20, 2022. Thus, although we could not test all households and potential contacts of enrolled neonates, we speculate that the high number of COVID-19 cases and the subsequent increase of potentially infected contacts might have favored the transmission of SARS-CoV-2 to neonates during the first month of life.

The current study has been conducted in the same area and with the same setting of previously published data that evaluated the rate of postnatal infection in neonates born to SARS-CoV-2-infected mothers during a pre-omicron era; therefore, these data and those obtained in the pre-omicron era can be compared. Moreover, this is the first large cohort in which the rate of postnatal SARS-CoV-2 infection has been investigated. Finally, the prospective design and consistent management of mother-infant dyads at the participating centers make our results confidently generalizable. However, we are aware of some limitations, such as the lack of sequencing data that could have confirmed the SARS-CoV-2 variant infecting each specific mother-infant dyad, even if we referred to regional SARS-CoV-2 sequencing data collected from the I-Co-Gen platform by the Italian National Institutes of Health. Other limitations could be the lack of parental/household contacts’ screening, the number of missed neonates at follow-up, and the unavailability of sterile neonatal samples to confirm or disprove the vertical transmission of SARS-CoV-2.

In conclusion, we demonstrate for the first time that neonates born to women infected by the SARS-CoV-2 omicron variant at delivery or in the first 2 days after delivery are at increased risk of infection during the first month of life compared with those born during the circulation of previous SARS-CoV-2 variants. Parental education on the correct application of preventive measures (wearing face masks, limiting contacts) should be reinforced both before and after the hospital discharge of the mother-infant dyad. However, as previously reported, most neonates infected by the SARS-CoV-2 omicron variant are asymptomatic, and rooming-in practices should still be encouraged with appropriate precautions.

COVID-19

coronavirus disease 2019

I-Co-Gen

Italian COVID-19 Genomic

IQR

interquartile range

SARS-CoV-2

severe acute respiratory syndrome coronavirus 2