COVID-19 immunity in family members reduces risk of infection by 45-97%

The COVID-19 pandemic has caused over 4.8 million deaths to date, with over 240 million reported cases. This sheer volume of human casualties, coupled with the damage to the world economy and the psychological stress of long-term restrictions on social interactions, have spurred an intensive effort to develop safe and effective vaccines against the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).

With the rollout of several vaccines based on adenovirus vector or nucleic acid platforms, over 48% of the global population has already received at least one dose of a COVID-19 vaccine. Despite these efforts, most of the world’s population remains at risk and is likely to remain so until 2023. In fact, some scientists estimate that at current rates, it may take up to five years to achieve 70-80% immunity.

The current study explores the dynamics of family transmission of the virus. The data from this study is expected to help determine vaccination strategies in situations of vaccine shortages.

The current study used data from Swedish national registries to compare transmission within families with varying numbers of immune individuals. Furthermore, the authors also used this information to compare the risk in settings with different sources of immunity including natural infection, one dose of the vaccine, or two doses (complete vaccination).
Study findings

The current study included over 800,000 families and approximately 1.8 million individuals. A family of two, neither of whom were immune, was the most common family; however, all configurations were identified. The sample was equally split between the sexes.

As the number of immune family members increased, the mean age went down compared to the non-immune individual at 27 and 52 years, respectively. The former type of family had fewer underlying conditions, a lower mean income, and a higher chance of being Sweden-born.

Over a total follow-up period of over 100,000 years, a diagnosis of COVID-19 was made in about 89,000 of non-immune individuals within a mean of 26 days. The number of immune family members went down with the risk of acquiring the virus.
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In families where only one member was immune, the risk of a non-immune family member getting the infection was 45% to 61% lower, regardless of the family size. With two immune members, the risk was reduced by 75% to 86%, whereas members in families with three immune individuals were protected against 91% to 94% of disease.

In five-member families with four immune members, the risk for the sole susceptible member was 97% lower. The risk did not change, even if a family member had severe COVID-19 and required hospitalization.

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