A high nasopharyngeal viral load – the amount of virus a person had in their nose and throat – would determine the likelihood of secondary transmission of the COVID-19, or SARS-CoV-2 virus within a household. According to a new study conducted by researchers at the University of North Carolina (UNC) School of Medicine, US, a high viral load could also indicate the severity of the disease should a household member take ill.

The UNC study, conducted between April and October 2020, followed 100 COVID-19-positive patients around the Raleigh, North Carolina area and included a total of 208 additional household members. A household member was defined as someone who was staying in the same living space as the person who tested positive (index case). Researchers tested household members with polymerase chain reaction (PCR) nasal swabs weekly for three weeks following the initial COVID-19 case, or by a seroconversion antibody test at the fourth week.

Excluding 73 household members who already tested positive for the virus when researchers got to their home, the secondary attack rate among household contacts stood at 32%. Most of the secondary cases were also noticed to occur within the first week of the initial positive COVID-19 test.

“We think this number is actually much higher,” said Dr. Jessica Lin, Assistant professor in the UNC Department of Medicine, Division of Infectious Diseases at the UNC School of Medicine. “Sometimes we were getting to households to test people four or five days after the index case showed symptoms. By that time a lot of household members were already infected. But because that infection happened before we got there, we couldn’t include it in our data.”

This study also took place before the more infectious Delta variant was widely circulating in the US, leading Dr. Lin to believe the current secondary attack rate in households is significantly higher.

Besides viral load, the study also looked at living density, essentially the concentration of people living within a household, as a factor that determined secondary transmission of the virus. Here, 44% of participants enrolled identified as Hispanic or non-white. Researchers found that minority households were more likely to experience a higher living density, and thus a higher risk of secondary infection than white households.

“It’s very difficult to follow public health guidelines in some living situations,” Dr. Lin said. “If you have multiple people and generations sharing common areas or bedrooms, or say you are a single parent, it becomes nearly impossible to isolate or even physical distance.”

Read: Face mask usage imperative to reduce risk of transmission of COVID-19

Dr. Lin says these findings all come back to one key message – vaccinations. The more people in a household that are vaccinated, the less likely the chance that secondary COVID-19 infections occur. Even if one person is vaccinated, it helps, especially if the vaccinated person happens to be the first infection in a household. A person who has been vaccinated will most likely have a lower viral load, which will make it harder for the virus to infect other household members.

“When you get vaccinated, you aren’t just protecting yourself, you’re protecting those important people around you.”

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