Someone in my house has COVID. How likely am I to catch it?

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

Given the contagious nature of COVID, especially newer variants, one would imagine that if you lived with someone who has COVID, you would inevitably become infected.

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

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

How is household transfer measured?

We use the “secondary attack rate” to describe the average number of secondary infections in a group of exposed people once a virus has been introduced into a given environment, such as a household. It takes into account a number of different factors, including:

• how contagious the virus is
• how high the viral load of the infected person is and how efficiently they shed the virus
• the vulnerability of others present
• the characteristics of the environment such as crowding and ventilation.

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

Since the pandemic began, we’ve also seen “superspreading,” where a small number of people account for a large proportion of new COVID cases.

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

Read more: How to prevent indoor COVID-19 superspreader events this winter

How has household transmission changed as a result of the pandemic?

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

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

The increase in infectivity of new variants emerging from late 2020 led to an increase in household transmission. The Alpha variant had a household secondary attack rate of 36.4%. This dropped to 29.7% for the Delta variant before rising again to 42.7% for the Omicron.

However, even studies as large and comprehensive as this are limited in their ability to make direct comparisons of all the factors that may affect secondary attack rates, such as: B. the household environment, the behavior of household contacts and the use of masks, to name a few. And that study didn’t include the newer Omicron variants.

Why has the household secondary attack rate changed?

The secondary attack rate for the Delta variant decreased compared to the Alpha variant despite its increased infectivity. This can probably be explained by increasing immunity in the population – both due to vaccination and due to previous infection.

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

Read more: No, vaccinated people are not “just as contagious” as unvaccinated people when 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 at just 42.7%. The increased immunity in the population is probably why it is not higher.

Vaccination reduces transmission

The reduction in the secondary infection rate in the household was greater when the households had their booster vaccination.

The bottom line is that sharing a household with an infectious person does not mean you will inevitably become infected, but full vaccination does help reduce the spread of Omicron among household contacts.

Read more: New COVID variants may be more easily transmissible, but that doesn’t mean the R0 — or basic reproduction number — has gone up