Similar viral loads in vaccinated and unvaccinated individuals with the COVID delta variant, suggest that both can transmit the virus.
There is now clear evidence that vaccination against COVID-19 reduces symptom burden and duration of any associated illness. An emerging problem is the appearance of new COVID-19 variants, in particular, the COVID delta variant. However, an analysis by Public Health England, has shown that two doses of a COVID-19 vaccine, reduces the incidence of hospitalisation in those infected with the delta variant. Furthermore, in the US, the Centers for Disease Control and Prevention (CDC) produced updated advice indicating how a body of evidence now suggests that full vaccinated individuals were less likely to either acquire COVID-19 or transmit the virus onto others. However, despite this, there are emerging concerns, particularly from Israel, which has managed to vaccinate a large proportion of the adult population, that the reported effectiveness of COVID-19 vaccines has reduced to 64% with regard to preventing infection and symptomatic illness. Interestingly, this reduced effectiveness appears to coincide with the spread of the COVID delta variant in the country. This is of particular concern given how it is thought that the COVID delta variant has an estimated 60% higher risk of household transmission. Thus, some uncertainty remains over the effectiveness of vaccines against the COVID delta variant and the extent to which the risk of transmission is reduced.
Potential for increased transmission
In light of this uncertainty, the CDC has released data captured from an outbreak of COVID-19 during July 2021, associated with large, public gatherings, in Massachusetts. During the month of July, 469 cases of COVID-19 associated with multiple summer events and public gatherings were reported. The reported vaccination coverage among eligible Massachusetts residents was 69% although the majority (74%) of symptomatic infected cases occurred in fully vaccinated individuals, who had received either the Pfizer-BioNTech or Jansen vaccine. Most infections (85%) occurred in males with a median age of 40 years.
A total of 133 patients provided samples for genomic analysis, of which, 89% were due to the COVID delta variant. Fortunately, only five people required hospitalisation (four who were fully vaccinated) and no deaths were reported. PCR cycle threshold (CT) values, which represent a measure of the concentration of viral load present in a sample, were taken from 127 vaccinated and 84 unvaccinated individuals. The lower the CT value, the more viral material present and the median CT values were 22.77 and 21.54 for the vaccinated and unvaccinated individuals respectively. In order words, given that the transmissibility of COVID-19 is known to be dependent on viral load, it would appear that the infection with the COVID delta variant, even among fully vaccinated individuals, could still lead to appreciable transmission of the virus. As a result of these findings, the CDC has now suggested that prevention strategies such as mask wearing should continue indoors, irrespective of vaccination status.