Wednesday , September 28 2022

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


An interesting new study published in the journal JAMA Internal Medicine Examines the risk of coronavirus disease 2019 (COVID-19) for a susceptible individual who is part of a COVID-19 immune family.

Study: Relationship Between Risk of COVID-19 Infection in Non-Immune Persons and Immunity to COVID-19 in Family Members. Image Credit: Chakkaphong Nutalay /


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

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

The current study explores the dynamics of familial transmission of the virus. It is expected that the data obtained from this study will help determine vaccination strategies in cases of vaccine shortage.

The present study used data from Swedish national registries to compare transmission in families with different numbers of immune individuals. In addition, the authors used this information to compare risk in settings with different sources of immunity, including natural infection, one dose of vaccine, or two doses (full vaccine).

Study findings

The current study included more than 800,000 families and nearly 1.8 million individuals. A family of two, neither of which was immune, was the most common; however, all configurations are defined. The sample is split evenly between the sexes.

As the number of immune family members increased, the average age relative to non-immune individuals fell at 27 and 52, respectively. The old family type had less basic conditions, a lower average income, and a higher chance of being Swedish-born.

Over a total follow-up period of over 100,000 years, approximately 89,000 people without immunity were diagnosed with COVID-19 within an average of 26 days. With the risk of contracting the virus, the number of immune family members has decreased.

Families in which only one member was immune had a 45% to 61% lower risk of contracting an infection by a non-immune family member, regardless of family size. With two immune members, the risk was reduced by 75% to 86%, while members of families with three immune members were protected against 91% to 94% of the diseases.

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

COVID-19 risk in families with two to five members.


This study was conducted in the context of the SARS-CoV-2 Alpha variant, which at the time accounted for over 95% of cases.

The researchers found that the risk of COVID-19 was reduced by 45% to 97% and was proportional to the number of family members who were immune to the virus. The source of immunity did not appear to matter. In this setting, vaccines appear to be important in preventing the spread of the virus within a family.

The greater the number of family members with a history of COVID-19, the lower the risk for susceptible family members. Thus, families with two of the three immune members saw the remaining susceptible members were at 86% lower risk, while the risk was up to 97% lower for families with three or four immune members.

In other words, the absolute risk of infection was proportional to the number of susceptible relatives in the family. Therefore, the susceptible family member had an absolute risk of infection of 3% to 5% in the largest family with four or more members.

Conversely, the relative risk reduction was greatest in large families. The absolute risk of infection therefore depends on the number of non-immune individuals in each family.

According to previous studies, one dose of vaccine provides protection against infection, illness, and death. However, it is unknown to what extent a single dose of vaccine can protect against familial transmission. The present study adds to the body of information by showing that a single dose appears to be as beneficial as two doses, or as beneficial as those obtained by natural infection.

This means that, in low-resource settings with vaccine shortages, these findings will help guide the vaccine where it is needed most. It is important to note that the single dose vaccine is less protective against the Beta and Delta variants.

More importantly, these variants are also more contagious than the Alpha variant, and the Delta variant dominates the current surge. The results of the current study need to be followed up with data on currently circulating variants.

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