How Rates of COVID-19 Infection Produce Data Spikes

As scientists track new cases of COVID-19, it’s important to understand that it takes several weeks after a policy change — such as a stay-at-home order or reopening businesses — for public health officials to see if the number of cases is rising or falling. Several cycles of infection, the spread of germs from person to person through a common series of events, need to occur before a noticeable increase or decrease will be reflected in the data.

Timeline showing two weeks of symptom onset
Infected person standing in a group
Three different social circles, showing subsequent groups infected from that first individual
Chart of climbing cases, with dotted lines moving in opposite directions
Declining case numbers when protective steps are taken
Increasing cases when safety precautions are ignored
  • When a person is exposed to the coronavirus, it can take up to two weeks before they become sick enough to call a doctor and get tested. If they test positive, their case is counted in the data.

  • But it may take another two weeks (or, 4 weeks after the initial exposure) for additional people to become ill after catching it from this person.

  • It takes even more time for data to show that additional groups of people become ill after being exposed to the first group. This is why it takes six to eight weeks after an event or a policy change to see the effect.

  • Several cycles of infection are needed before a detectable increase appears in the data. A similar timeframe is needed to detect a decrease in overall cases after implementing a new policy like universal masking.

  • In areas where community transmission is low and everyone continues to wear masks, wash their hands, and practice physical distancing as things reopen, the number of cases of COVID-19 will likely stay low or continue to decline.

  • If communities reopen and safety guidelines are not followed, surges will likely occur six to eight weeks after reopening. Please be sure to follow all safety guidelines to protect yourself and others.

Coronavirus (COVID-19)

Scientist carefully insets a pipette into a test tube.

What you need to know from Johns Hopkins Medicine.