Someone in my house has COVID. What are the chances that I will get it?

During the pandemic, one of the biggest COVID risks was sharing a home with someone who is contagious.

Given how contagious COVID is, especially more recent variants, you would imagine that if you live with someone who has COVID, it would be inevitable that you would become infected.

But this is not the case. A recent study suggests you have a 42.7% chance of contracting COVID from a roommate who tests positive for Omicron.

That means that if someone introduced the Omicron variant to a household of six, you would expect that on average two of the remaining five members of the household would become infected.

How is the transfer of households measured?

We use the ‘secondary attack rate’ to describe the average number of secondary infections among a group of exposed people, once a virus is introduced into a particular environment, such as a household. It is responsible for a number of different factors, including:

  • how contagious the virus is
  • how high the viral load of the contagious person is, and how efficiently they shed the virus
  • the sensitivity of other attendees
  • the characteristics of the environment such as crowds and ventilation.

The secondary attack rate is an average and transmission varies considerably between households. Thus, some households see that all members are infected, while others have little or no transmission.

From the start of the pandemic, we have also seen “superspreading”, where a small number of people are responsible for a large proportion of new COVID cases.

Conversely, a large proportion of infected people do not spread it at all.

Read more: How to avoid indoor COVID-19 ‘super spreader’ events this winter

How has the transfer of households changed due to the pandemic?

A meta-analysis (merging results from previous studies) published in April combined results from 135 studies and 1.3 million people in 136 countries, published in the Journal of the American Medical Association (JAMA).

It estimates that the secondary household attack rate for the original virus was 18.9%. So your risk of getting infected with COVID if you shared a home with one or more infected people was about one in five.

Person lies on the couch in a hoodie, under a blanket.
Some people don’t spread COVID at all.
Rex Picker/MONKEY

The increase in the infectiousness of new variants that emerged at the end of 2020 translated into an increase in household transmission. The Alpha variant had a secondary household attack rate of 36.4%. This fell to 29.7% for the Delta variant, before rising again to 42.7% for Omicron.

However, even such large and comprehensive studies are limited in their ability to make direct comparisons of all factors that can influence secondary seizure rates, such as the household environment, the behavior of household contacts and the use of masks, to name but a few. to name. . And this study did not include the newer Omicron variants.

Why has the number of household secondary attacks changed?

The secondary attack rate for the Delta variant decreased compared to the Alpha variant, despite its increased infectivity. This is probably explained by the increasing immunity of the population – both from vaccination and from previous infection.

Although vaccines were not as effective against Delta as previous variants, and the protection decreased over time, they still reduced the risk of transmission from households.

Read more: No, vaccinated people aren’t ‘as contagious’ as unvaccinated people if they get COVID

Despite a significant increase in the infectivity of the Omicron variants and their immune-escape properties, the risk of becoming infected in a household was still estimated to be only 42.7%. Increased immunity in the population is probably the reason it is not higher.

Vaccination reduces transmission

The decrease in the secondary seizure rate of households was greater when households had received their booster vaccination.

The benefit is that sharing a household with an infectious person does not mean you will inevitably become infected, but being fully vaccinated will help reduce the spread of Omicron among household contacts.

Read more: New COVID variants may be more transmissible, but that doesn’t mean the R0 – or base reproduction number – has increased

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