Reaction to study analyzing delta transmission in British homes

Reaction to study analyzing delta transmission in British homes

This article was published on
November 3, 2021

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This article is also available in Spanish.

This article is also available in Spanish.

An analysis of 602 community contacts of 471 covid-19 index cases indicates that the secondary attack rate (SAR) for household contacts exposed to the delta variant is 25% in fully vaccinated individuals compared to 38% in unvaccinated individuals.

An analysis of 602 community contacts of 471 covid-19 index cases indicates that the secondary attack rate (SAR) for household contacts exposed to the delta variant is 25% in fully vaccinated individuals compared to 38% in unvaccinated individuals.


Community transmission and viral load kinetics of the SARS-CoV-2 delta (B.1.617.2) variant in vaccinated and unvaccinated individuals in the UK: a prospective, longitudinal, cohort study

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What our experts say

Context and background


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Expert Comments: 

Óscar Zurriaga

The article attempts to explain how and why the delta variant is transmitted as efficiently as it is in populations with high vaccination coverage.

It is a population-based cohort study based on PCR-confirmed index cases of COVID-19 with symptoms. Contacts were identified, aged over 5 years, notified within five days of the onset of symptoms of the index case. They were categorized into household and non-household contacts.
The conclusions are directed in two directions: differences in secondary attack rates (SARs) [a measure of transmissability] in the groups of household and non-household contacts and according to their vaccination status, and viral loads between these groups and their evolution over time.

The results are congruent with what has been published regarding the ability of COVID-19 vaccines to prevent or not the transmission of infection. The article adds quantitative value to this result in a specific setting such as households, where personal interaction is closer and more prolonged, which has clear public health implications for transmission control.

It should be noted that the main limiting factor and possible confounder is age. Probably due to the vaccination strategy in the United Kingdom, which is similar to that of other countries, the age of the study participants is lower than that of the non-vaccinated groups infected with the delta variant. Age, on the other hand, also plays an important role in viral load peaks.

The authors counter this limitation by arguing that the higher secondary attack rate observed in unvaccinated contacts is unlikely to have been due to younger age rather than lack of vaccination. They add that, on the other hand, there is no published evidence showing increased susceptibility to SARS-CoV-2 infection with younger age. Perhaps, if the number of cases (indices and contacts) collected were greater, and the circumstances different, this limitation could have been overcome, but it is difficult to do so in the contextual circumstances in which the study was conducted (vaccination by decreasing age groups and increasing vaccination coverage at the same time as the follow-up period developed).

The authors' main conclusion is that, although vaccination reduces the risk of delta-variant infection and accelerates viral clearance, fully vaccinated persons with post-vaccination infections have a peak viral load similar to that of unvaccinated cases and can efficiently transmit infection in household settings, even to fully vaccinated contacts.

Given that the home environment may represent a setting of increased personal interaction, the authors themselves emphasize that protection should be direct through, primarily, vaccination and non-pharmacological interventions, which remain essential. The results presented in the article show this. And in the field of vaccination, booster programs and adolescent vaccination are also pointed out as aids to increase the currently limited effect of vaccination on transmission.
It represents a wake-up call towards the elimination of measures, even in areas of interaction of fully vaccinated individuals, although it is true that the home environment may be the least likely to sustain them.

It is essential to carry out population-based or community studies that are closer to what happens in reality and provide results that have implications for action and intervention in the health of populations.

Mario Fontán Vela

This article analyzes the secondary attack rate (SAR) [a measure of transmissability], in a context of dominance of the delta variant in the UK, and the viral dynamics in a sample of participants recruited through the tracking system available in the country. Within the limitations of the article, the design allows us to analyze the dynamics of transmission with a new variant and taking into account the vaccination status of the contacts of the index cases. However, due to its limitations (small sample, in specific regions of the UK, sample selected from declared contacts, etc.), the conclusions are somewhat definitive for the design's ability to answer the questions posed.

On the one hand, the results are consistent with other studies that also observe a lower SAR among vaccinated contacts, which reinforces the evidence we have on the ability of vaccines to reduce the possibility of infection. However, the results obtained in relation to the ability of an index case to infect depending on the vaccination status (same SAR in vaccinated and unvaccinated) contradict some studies that have seen a lower transmission capacity if the index case was vaccinated ( and and However, although the evidence would seem to indicate that the reduction in transmission exists, more studies are still needed to have more conclusive data.

The authors acknowledge some of the important limitations, such as the recruitment of participants through the tracking system. They also mention that some groups in the study differed by the prioritization of vaccination in the older groups. However, they state that the fact that young people are the most infected by Delta and without vaccination does not influence the SAR, when the patterns of socialization are different depending on the age groups, especially in the summer period where the information of patients infected with Delta is concentrated. I think their statement is somewhat blunt in this regard. On the other hand, they do not mention that, as the participants are concentrated in certain regions, there may be biases related to social behaviors or living conditions that influence the data analyzed.

The implications of this study are limited and I believe that it will fundamentally serve to incorporate more information in relation to the dynamics of transmission, but conclusive evidence cannot be inferred from what is provided in this article, taking into account that there are other studies that provide evidence to the contrary. I consider that it is useful to have more information and to be able to contrast with other studies, but not at this time and due to the limitations mentioned, little further.


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