Are breakthrough infections milder on average?

Update August 10, 2023: The findings reported here were published as part of a more extensive analysis in the scientific journal PNAS Nexus.

Dear Donors,

in a previous blog post we showed how our vitals are expected to change over the course of a Sars-CoV-2 infection. We saw elevated resting heart rate, a large reduction in daily activity and prolonged sleep duration in the weeks following a positive PCR–test, all of which can be interpreted as signs of illness and an increased need for rest.

Thanks to your active participation in the data donation project and the corresponding surveys we now know the vaccination status of 25255 donors and the results of 16489 PCR tests (2149 of which were positive). Combining these data points allows us to discriminate between infections that took place before and after donors were at least fully vaccinated. For both groups (vaccinated & unvaccinated) we investigated expected changes in donated vital data compared to a two month period prior to an infection, i.e., typical values of sleep duration, activity and resting heart rate when users have, most likely, been healthy. This way, we hope to see what effects vaccination may potentially have on the long-term effects of COVID-19 that we observed in our earlier report.

A word of caution before we dive into the analysis: By today almost all donors that provide vaccination status are either fully vaccinated or boostered. Whenever we talk about unvaccinated cases in the following we refer to users that caught COVID-19 prior to their vaccination. This is most likely because vaccines were not yet readily available by the time of infection of those donors. Almost all unvaccinated cases have been recorded in late 2020 and early 2021 (the second and third wave) while most breakthrough infections are recorded from fall 2021 onwards (the fourth and fifth wave). In this sense it is possible that the results below might also partially arise from distinct physiological responses to the different variants of Sars-CoV-2.

Resting heart rate changes are less severe for vaccinated donors

Let’s start by investigating expected changes in resting heart rate (RHR) around the time of a positive COVID-19 test. For unvaccinated COVID-19 positive donors (red curve in Fig. 1) RHR starts to increase in the week prior to taking the test, which might be a precursor to an upcoming disease. The largest change is observed in the week of taking the PCR-test with an expected value of around 2 beats per minute above typical values. The normalization (indicated by a change of around zero) in week one has been observed in other studies as well and might be linked to what is called Bradycardia – a potential symptom of COVID-19. After this short period RHR increases again and then slowly returns to its normal value over a long course of 13 weeks.

In contrast, RHR-changes of vaccinated donors (shown in purple in Fig. 1) are comparatively smaller, with an increase of only around 1 beat per minute in the week of taking the test and a much faster return to normal values after at most 7-8 weeks. This implies that both, the severity of an infection in the acute phase reduces and the recovery process improves for vaccinated donors. Note that we can only display vital changes for vaccinated donors up until 9 weeks after an infection, since most cases have been reported from November 2021 onwards.

Activity of vaccinated donors likely only decreases during quarantine

A second metric that we often analyze in the data donation project is daily activity measured in terms of step count. For both,vaccinated and unvaccinated donors, this value decreases strongest in the week following a positive PCR-test, most likely due to staying at home as people isolate themselves. However, we note two subtle, yet important, differences between vaccinated and unvaccinated cases:

(i) The activity reduction is almost twice as strong for unvaccinated donors in the week of taking the PCR-test. Hence decreases in activity likely happen much earlier than for vaccinated donors, probably already in the days prior to taking a test. It is reasonable to assume that such a reduction is already indicative of an upcoming illness and not only a response to quarantine orders which might not be related to a person’s actual health status.

(ii) While the activity of vaccinated donors mostly returns to typical values after three to (at most) six weeks we find small deviations from such typical values for unvaccinated donors that only vanish after 13 weeks. This implies that the risk of prolonged reductions in the ability to go about usual activities beyond isolation periods is larger for unvaccinated people, albeit not vanishing entirely for vaccinated people either.

Sleep duration increases during quarantine

Finally, we investigate a third marker for a person’s health status given by the typical sleep duration. For unvaccinated cases we find an increase in sleep duration of about 40-50 minutes per day in the week of taking a PCR-test and the week thereafter. Again, this increase is visible for vaccinated donors as well, even though the magnitude of this effect is more than halved. Interestingly, the sleep duration in vaccinated cases mostly increases in the week after taking a COVID-19 test, thus could again be a result of people isolating and thereby spending more time at home. In contrast, the sleep duration of unvaccinated donors already largely increases in the same week of taking the test, which might again be indicative of a larger need for rest due to health problems caused by the Sars-CoV-2 infection.

Summary

In sum, we find that short- and long-term changes in the vital data of COVID-19 positive donors are less pronounced for vaccinated cases. This ties in with commonly observed results that indicate a milder course of breakthrough infections compared to those recorded in people that have not yet achieved some form of immunity. At the same time we find that vital signs return back to normal values faster, thus hinting that vaccinations might also mitigate prolonged health problems such as Long-Covid.

As always, we express our greatest gratitude to you, our donors, for making such analyses possible. Thanks to your active participation we understand more and more about the effects of COVID-19 on our physiological conditions and how likely and severe long-term effects potentially manifest. We are currently planning to launch a new survey in the App with a specific focus on these topics and we already invite you to stay tuned about its launch.