SARS-CoV-2 Delta infections among vaccinated healthcare workers


Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) genetic lineages have wrought havoc by mutating and circulating around the world. The B.1.617.2 (Delta) variation is the most frequent lineage in India and around the world. Furthermore, breakthrough infections with the Delta variant have been reported in COVID vaccination recipients and people who had previously been infected with SARS-CoV-2. It’s uncertain whether people who have received two doses of COVID-19 vaccine are entirely immune to the virus.

The Delta variant’s vaccine efficacy (VE) for mild symptomatic infection is anticipated to be around 50% after two doses of either mRNA or adenovirus-based vaccination. Previously, 155 cases of vaccine-related illnesses were reported among frontline Delhi healthcare workers (HCW), the majority of whom had received one or two doses of the Covishield (ChAdOx1) vaccination and were immunized >21 days prior to the symptomatic presentation.

According to known data, individuals in both vaccinated and unvaccinated groups had identical viral loads, although the former group had a steeper viral decay over time. In addition, inoculated people were less likely to have a viral culture. It’s unknown if those who have a vaccine breakthrough distribute it to other vaccinated people or if unvaccinated people are the ones who drive the transmission.


The Research

A new study published on the medRxiv* preprint server used in-silico approaches to assess likely Delta-variant transmission events between vaccinated healthcare workers (HCWs) in Indian hospitals, using genomic and epidemiological data.

The participants in the trial were frontline healthcare workers who had SARS-CoV-2 symptoms and were tested for viral presence using reverse transcriptase-polymerase chain reaction (RT-PCR).

Consensus sequences in fasta format were obtained from two different hospitals in Delhi, India, for bioinformatics and phylogenetic research.

SARS-CoV-2 Delta infections among vaccinated healthcare workers
HCW transmission networks that could exist. Individual labels are colored according to immunization status, including the date of the second shot prior to illness, if applicable. The probability of different pairwise transmission events between individuals is represented by the thickness of lines between individuals; these probabilities are conditional on transmission occurring between the individuals seen in each network. The labels represent the dates on which individuals first became sick, as well as the nucleotide gains in sequences taken from each individual in comparison to the mutual consensus.

The result

In hospital A, there were 81 breakthrough infections among 1,100 HCWs, while in hospital B, there were 32 breakthrough infections among 4,000 HCWs.

In doubly vaccinated HCWs, B.1.617 was detected in 90.7 percent of breakthrough infections. B.1.1.7-like bacteria made up 5.3 percent of the total, whereas B.1.538 bacteria made up 1.3 percent.

The most common symptoms were fever (82.3 percent of all cases), cough (43.4 percent), myalgia (20.4%), and loss of smell/taste (14.2 percent). In hospital A, 66 sequences provided more than 90% high-quality whole-genome coverage, including 43 cases of breakthrough infection. In hospital B, 52 samples were used to produce high-quality genome sequences, including all 32 symptomatic breakthrough infections.

There were several introductions into hospitals A and B, followed by intra-hospital transmission. As a result, the projected phylogeny was found to be significantly partitioned, splitting the sequences into separate clades.

The Delta consensus mutations that were reported in these cases were found everywhere over the SARS-CoV-2 genome. In homoplasic sections of the genome, neither of the single-nucleotide polymorphisms (SNPs) was discovered.

Individuals 115, 127, and 305 in hospital A had viral genomes that were all identical. In all, 1,381 plausible transmission networks involving various medical staff were found in hospital A, while 128 probable transmission networks involving various medical staff were identified in hospital B.

The transmission network identified for hospital A was determined to be self-consistent when the data was compared to simulations. The network predicted for hospital B, on the other hand, may be skewed by missing data, given most patients have unique mutations. A junior medical staff member, a nursing student, and a nursing staff member at hospital A – who developed sickness over six days – shared a same genome sequence, but three other team members had viral genomes that differed by a single SNP. In addition, a junior medical staff member had the nonsynonymous substitution T3082I in ORF1ab.

The current study has two major limitations: first, it only looked at symptomatic individuals, eliminating those who were asymptomatic or who failed to report their infection status. Second, the lower number of valid samples resulted in transmission chains that were incomplete.

Despite vaccinations’ high efficiency in avoiding severe disease, the current data point to the likelihood of Delta variant transmission between fully vaccinated persons in a healthcare context. Furthermore, breakthrough infections were identified in hospital settings in people who received the second vaccine dose within 60 days of the peak of circulating neutralizing antibody levels.

This study emphasizes the significance of maintaining infection control measures, even in highly vaccinated populations, and the usage of masks in people who have had two doses of vaccine to prevent onward transmission.

*Important Notice

Preliminary scientific reports published on medRxiv are not peer-reviewed and should not be regarded as conclusive, should not be used to influence clinical practice or health-related behavior, and should not be recognized as established information.