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Transmission event of SARS-CoV-2 Delta variant reveals multiple vaccine breakthrough infections

Our take —

This study, available as a preprint and thus not yet peer-reviewed, investigated six symptomatic COVID-19 cases in vaccinated individuals after an outdoor wedding in Houston, Texas in April 2021. All patients were infected with the Delta variant, and phylogenetic analysis confirms likely transmission during the wedding event. However, limited data on time since vaccination (or antibody titers) for these patients makes it difficult to confirm how many were true vaccine breakthroughs, and the total number of asymptomatic infections acquired at the wedding is unknown. Breakthrough infections should continue to be monitored, especially as Delta or other new variants increase in frequency. However, it is important to remember that vaccine breakthroughs are to be expected and do not suggest vaccine ineffectiveness; in fact, vaccination may have protected some of these patients (and other wedding guests) from more severe outcomes.

Study design


Study population and setting

This study describes SARS-CoV-2 infections acquired at a wedding in Houston, Texas with 92 attendees in April 2021. All wedding activities occurred outside in a large open-air tent, and attendees were required to be fully vaccinated. Two guests (Patients 0a and 0b) travelled from India to attend the wedding and tested positive four days after the wedding. Patient 0b complained of fatigue the first night of the wedding but associated it with jet lab. Patient 0a developed a cough two days after the wedding, and both developed a fever three days after the wedding. They had both received their second dose of the Covaxin BBV152 vaccine 10 days prior to traveling to Texas. Five other guests reported close contact with patients 0a and 0b, and four tested positive for SARS-CoV-2. Two had been vaccinated with the Pfizer vaccine, and two had been vaccinated with the Moderna vaccine, though vaccination dates for these patients were not reported. All six positive patients reported symptoms, and at least two required hospitalizations. Patient 0a was one of those hospitalized, and he died approximately one month after the wedding from complications of COVID-19.

Summary of Main Findings

Whole genome sequencing revealed that all six cases were caused by the SARS-CoV-2 Delta variant. Phylogenetic analysis revealed that the cases were closely related (viruses formed a distinct cluster within the Delta clade of the phylogenetic tree). The authors characterize all six infections as vaccine breakthroughs due to reported vaccination status of all patients.

Study Strengths

In this study, the phylogenetic and epidemiological data both suggest the same conclusion: that transmission of SARS-CoV-2 occurred at a wedding attended by all six patients. The study also provided detailed information on patient demographics and comorbidities, and includes travel history of two patients from India, indicating the likely source of initial infection.


The manuscript does not provide any details on testing of other wedding attendees, including those that may have been asymptomatic, limiting our understanding of the true number of attendees that may have been infected. Additionally, the manuscript does not discuss the severity of symptoms in patients other than 0a and 1. Finally, the paper states that Patients 0a and 0b received their second vaccine dose only 10 days prior to travel, which may have not been long enough for them to mount a full immune response. Therefore, only up to four of these cases may have been true vaccine breakthrough infections (though time since vaccination was not reported for the other patients).

Value added

This manuscript presents additional evidence of symptomatic infections — particularly of the Delta variant — in vaccinated individuals.

This review was posted on: 24 July 2021