Scientists from Florida, USA, have conducted a study to assess the likelihood of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) transmission in individuals with delta breakthrough infections. The results show that the viral load in cases of delta breakthrough is lower than unvaccinated individuals. The viral load in the majority of delta cases is higher than the threshold for transmissibility. The study is currently accessible on the MedRxiv* preprint server, pending peer review.
Numerous new variants of SARS/CoV-2 are emerging as a result of the 2019 coronavirus pandemic (COVID-19). Due to the increased susceptibility and pathogenicity, several of these variants have been classified as Variants of Concern (VOCs) by the World Health Organization (WHO).
The SARS-CoV-2 delta VOC first was identified in India, contains multiple spike mutations that make the virus more contagious and resistant to infection or vaccination-induced immunity. In the US, there has been a dramatic increase in delta cases and hospitalizations since June 2021, despite the high level of vaccination. This indicates a decline in the vaccine’s immunity against the delta variant. Furthermore evidence suggests that the majority of delta cases are linked to high viral load, which suggests greater viral transmission capacity.
The current study is examining whether the delta variant of the virus is associated with a higher prevalence of vaccine-related breakthrough infections and whether individuals who have been fully vaccinated against the virus are able to transmit it to others. Additionally, they have estimated the proportion of delta-related breakthrough cases that have viral loads greater than the transmissibility threshold during acute infection.
The researchers have sequenced 4,439 genomes of SARS-CoV-2. This is about 23 percent of the cases that were detected between October 2020 in Alachua County and July 2021. Of these of which 109 were vaccine breakthrough cases. They conducted a series of statistical analyses to examine the relationship between viral load and transmission in the cases of delta infection that were breakthrough. To test the possibility of direct viral transmission between people who have been vaccinated they carried out contact tracer and phylogenetic analysis.
Florida witnessed a dramatic increase in the number of cases of vaccine breakthrough due to the delta variant in July 2021. Of the 109 cases of breakthrough infections identified, 58 were caused by the delta variant. The average duration between full vaccination and infection detection was three months. Compared to vaccinated people, a relatively higher frequency of delta infection was observed in people who were not vaccinated.
A large majority of breakthrough cases were caused either by transmissions at the household level, or at the community level.
Viral load in delta infection
People who were vaccinated against delta infection had 38% less viral load compared to unvaccinated people with delta infection. The same was true for those with no delta infections. However 59% of delta-infected vaccine breakthrough cases had a viral load that was above the threshold for transmissibility.
Over time, distribution of SARS-CoV-2 variants in Alachua County (Florida) (A) Lineage distribution (y-axis) against. time (x-axis) among sequenced samples. The black, dotted line (right-y-axis) is the total number of samples successfully sequenced. (B) Lineage distribution (y axis) in relation to. the time required in cases of vaccine breakthrough. Total number of samples successfully sequenced is shown by the red drawn line (right y-axis). (C) number of cases that have been identified as breakthrough (x-axis) against. time between 2nd vaccination dose and diagnosis (y-axis).
Viral transmission between vaccinated people
The study identified six transmission pairings through contact tracer analysis and phylogenetic analysis. Each pair comprised a fully vaccinated recipient and donors with delta infection. The donors had been fully vaccinated with COVID-19-based mRNA vaccines for at least 120 days. Six of the patients had also received the mRNA vaccine for the course of 67 to 164 days. Overall, these observations indicate that direct transmission of viruses can occur between people who have been fully vaccinated delta infections.
Further analysis revealed that potential for transmission of viruses in delta-induced breakthrough cases doesn’t depend on the type of vaccine or the time since full vaccination. There was no relationship between viral loads and vaccine type, nor time between symptom onset, sampling or the time since complete vaccination.
Reconstruction of phylogenetic relationships and transmission characterization of SARS-CoV-2 Delta Delta sequences collected from Alachua County, Florida, and other locations that are relevant to the epidemiology. (A) A sequence phylogeny with a heatmap showing cluster origin, geographical origin , and vaccination status for each sequence. (B) Phylogenetic relationships of sequences from donor(D)-recipient(R) pairs linked via exposure using contact tracing (branches are colored according to cluster origin). Bootstrap support (BS) within clades that contain transmission pairs that are >90% are represented by black dots. Individuals with confirmed no known travel within 2 weeks prior to diagnosis are represented with Asterisks. Branches are scaled in substitutions/site.
The significance of study
The results of the study reveal that COVID-19 vaccines do not reduce the risk of person to person SARS-CoV-2 infections even after a complete vaccination. Researchers have stated that further research is needed to determine if an additional dose of booster vaccine is effective in controlling new infections and the transmission.
medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.
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