SARS-CoV-2 alpha variant exhibits enhanced household transmission

Household transmission remains a major driver in the spread of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) throughout communities worldwide. While a number of reports have described the intra-household transmission of SARS-CoV-2, few have compared transmission rates between SARS-CoV-2 variants of concern (VoCs) in this setting.

In a paper recently published on the preprint server medRxiv*, the transmission rate of wildtype and the Alpha variant of SARS-CoV-2 through households is tracked. In this study, the researchers find that the risk of secondary infections is significantly heightened amongst residents with VoCs.

Study: Household transmission of SARS-CoV-2 in Norway; a prospective, longitudinal study showing increased transmissibility of the Alpha variant (B.1.1.7) compared with other variants. Image Credit: Evegeny Atamanenko / Shutterstock.com

How was the study performed?

Sixty-five households in Norway with confirmed coronavirus disease 2019 (COVID-19) cases by reverse-transcriptase polymerase chain reaction (RT-PCR) between May 2020 and May 2021 were included in the study. Participants were refined to fit the following criteria, in which only one person who was at least 12 years old in the household had initially tested positive for COVID-19. Additionally, no other individuals in the house were vaccinated.

Each of the selected households was visited eight times for oropharyngeal and saliva sample collection over six weeks. These samples were subsequently tested for SARS-CoV-2 by RT-PCR, with positive samples sent for further immunological analyses to identify the virus lineage.

Questionnaires related to the household were also completed by each participant. These surveys included a wide range of questions on COVID-19 risk factors, both medical and behavioral. Survey participants were also asked to describe any quarantine precautions that they or other individuals within the household had undertaken.

Of the 65 primary COVID-19 cases included in the study, 18 were found to be positive for the Alpha variant, one for the beta variant, and the remaining positive for either the wildtype strain or another non-VoC. Notably, all Alpha variant cases were recruited in the latter months of the study, thus reflecting the changing viral population in the community over the course of the pandemic as the more highly transmissible Alpha strain became dominant.

In every reported case where initially non-infected household members contracted COVID-19 during the study, the specific genetic lineage was found to match that of the primary case. It is therefore likely that the majority of these infections originated from within the household.

The group also note that the average age of primary cases was greater than amongst secondary cases, 38 years compared to 24 years. It should be noted that this observation is likely an artifact of study design, as secondary but not primary cases aged 3-11 years old were included.

Differences in household transmission between SARS-CoV-2 variants

In total, 200 participants who were divided among 65 households were involved in the study, of which 132 individuals (66%) eventually became infected. Of those infected, almost half had mild or moderate symptoms, with just 14% being asymptomatic at two weeks post-infection.

Children were more likely to be asymptomatic than adults. Additionally, a higher percentage of asymptomatic cases were reported amongst those with the Alpha variant (22%) as compared to any other non-VoC (9%).

The authors found that secondary transmission occurred in 60% of all households included in the study. Approximately half of all initially non-infected individuals contracted SARS-CoV-2 during the six-week course of testing.

Upon differentiating between households with the Alpha variant, the authors of the current study found that secondary transmission occurred in 83.3% of Alpha-households, as compared to 55% of households who tested positive with other strains of SARS-CoV-2. Similarly, 77.8% of initially uninfected individuals in Alpha lineage households became infected, while only 42.5% of individuals became infected in non-VoC households.

The authors of the current study discount a number of factors as irrelevant to transmission rates within the study. Moreover, no discernible difference based on sex or blood type was observed, though older adults were more likely to become infected.

Household size was also disregarded as a major factor. Although “overcrowded” houses tended to bear a greater infection risk between individuals, this was often offset by the smaller number of individuals sharing such a space.

Overall, no difference was noted in secondary transmissions relating to the type of house, house size, or the number of bathrooms. There was, however, a trend towards more probable transmission if the household members shared a toilet, came into touch contact often and shared a bed with the primary case before infection was noted.

The only household-based non-pharmaceutical intervention that was found to be efficacious was avoiding bed-sharing. To this end, 38% of those who slept separately contracted SARS-CoV-2, as compared with 67% of bed-sharers.

*Important notice

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.

Journal reference:
Michael Greenwood

Written by

Michael Greenwood

Michael graduated from Manchester Metropolitan University with a B.Sc. in Chemistry in 2014, where he majored in organic, inorganic, physical and analytical chemistry. He is currently completing a Ph.D. on the design and production of gold nanoparticles able to act as multimodal anticancer agents, being both drug delivery platforms and radiation dose enhancers.

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