What does asymptomatic mean? Takeaways from UCSF’s Mission Study
Previously symptomatic means that the individuals had symptoms prior to being tested but not when they were tested.
Pre-symptomatic means individuals had symptoms after being tested but not when they were tested.
About the Study
In the preliminary results from the UCSF Mission census tract study, 83 people tested positive for COVID-19. In these very preliminary results, released a month ago, the study just divided people up into two categories: either symptomatic (40 people - 48%), or asymptomatic (43 people - 52%). It was eye opening to see that there were so many asymptomatic people with COVID-19.
The researchers followed up with the people who tested positive to further refine these two categories into the following four:
Symptomatic at the time of the test
Previously symptomatic prior to the test. (The researchers asked people whether the recalled having symptoms prior to the test.)
Presymptomatic (The individuals were asymptomatic at the time of the test but later developed symptoms.)
Truly asymptomatic (People who didn’t have symptoms prior to the test, at the time of the test, or after the test.)
The bar graph at the top of this post shows the divisions between the original simple classification and the newer, more complex classification.
People being people, there is also a small category of people who the researchers couldn’t follow up with for whatever reason. Thus, the original categorizations contained 83 individuals, while the new categorization contains only 80 people.
One person who tested positive needed to be hospitalized.
Not Feeling Sick? Wear A Mask Anyway.
Even asymptomatic people have high virus levels.
The graph above shows how much virus is present in the swab sample taken from an individual. The assumption is that the more virus there is the more infectious that individual is. The researchers then combined the swab results with the antibody results for those individuals and the follow up information. This allowed them to look at viral load amongst people who were symptomatic, pre-symptomatic, and truly asymptomatic.
First, notice that the blocks are split into antibody negative (Ab-) and antibody positive (Ab+). Any individual who has an active infection and has developed antibodies has a much lower viral load and presumably is less infectious. All of the results circled in red are lower than the other results immediately to their left that are antibody negative tests.
Second notice that there is a large group of the asymptomatic individuals who are still shedding lots of SARS-CoV-2 virus and presumably are still quite contagious. This is the green circled group of points.
The key take away here is: Please wear a mask even if you aren’t feeling sick.
Herd Immunity? Nope.
COVID-19 prevalence in a hard hit census tract is estimated to be at 6.1%.
There have been several theories circulating that COVID-19 has swept through the Bay Area. I have heard lots of people hypothesizing that they’ve already had COVID-19. By grouping all people together who are either swab positive (ie actively infected), or antibody positive (ie previously infected), this study can estimate the fraction of people in this particular community who have been exposed to the virus.
The actual number in the test was 3.1% of the population who has been exposed, but the researchers then adjusted that initial number to take into account the idea that many of the people who might be at most risk, might not show up to get tested. (ie their sample was biased.) After doing this adjustment, the researchers estimate that 6.1% of this hard hit census tract has been exposed to the virus.
The key take away here is: Even this highly impacted community is far away from herd immunity.
Economic disparity between early outbreak and later outbreak.
Compare the two groups of red circled numbers.
This study and San Francisco’s ethnicity data have shown how uneven the impact of this virus is. This uneven impact is mirrored in the economic data too. One of the fascinating things you can see in this study is how the impact of COVID-19 has shifted between the earliest outbreak and its current spread.
In particular, in this study no-one with an income over $100k tested positive with an active infection. However, the antibody tests do indicate prior exposure to COVID-19. Positive results in these tests tells us where the virus had been in the earlier stages of the outbreak. And in these tests, there were a number of positive results for people with incomes over $100k.
As my wife put it, what probably happened is that wealthy individuals are more mobile. They introduced COVID-19 and contributed to the initial spread of the disease, but then these same class of people were able to socially distance much more effectively. The spread of COVID-19 in the upper income brackets stopped, but the spread continued in the lower income brackets.
Original paper, feedback, and questions
Here is a link to the original paper if you want to read it for yourself https://www.medrxiv.org/content/10.1101/2020.06.15.20132233v1.
Since these results were so interesting, I rushed to get this post out. That also means that you might have questions that I haven’t answered, or you might catch problems that I overlooked. Please feel free to ask questions through the submission form. I’ll do my best to answer them.