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Illness Is Spreading Unseasonably Fast

September 11, 2021  ||  by Matt Albasi

Something odd is happening in homes across the country — Infectious illness is spreading unusually fast for this time of year. 

Household Secondary Attack Rate, or SAR, measures how transmissible an infectious disease is within a household. In other words, it’s a measure of how likely someone else in the home is to get sick when one family member becomes ill. 

Kinsa data shows that right now, SAR is as high as during the peak of flu season, despite there being very little flu. Two main factors likely drive this abnormal peak in SAR:

Line chart showing SAR since 2017. 2021 has the highest value in September of all the years
  1. Delta variant. According to the CDC, this variant is more transmissible than previously circulating strains. Since a more infectious virus is likely to infect more people in a household, an increase in SAR would follow.. But that isn’t the only thing driving the metric up.

  2. People’s weakened immune systems. After more than a year of pandemic precautions, immune systems are “out of practice” with regular illnesses. Because of this, people are more susceptible to a less-virulent virus simply because their bodies aren’t ready to protect against it. This would cause SAR to increase as well.

Determining the exact cause of this peak requires a lot more study. Kinsa’s data and epidemiology teams are continuing to monitor the trend in Household Secondary Attack Rate closely and will report any interesting findings as we learn them. 

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