Covid 19. How many people are actually infected? Santa Clara County

One of the key questions that remains unanswered about Covid 19 is how many people have actually been infected.  To date, most of the people tested have had some kind of symptom.  More recently, there have been more asymptomatic patients tested but the results are still pretty sparse.  The issue is tha fatality rate (the number of people who die if they get infected) will be greatly overestimated if large numbers of patients have no or only mild symptoms.  Some have speculated that at the number of people who are infected and don’t know it are many times higher than the number that have actually tested positive.  It is hoped in the coming weeks, when antibody tests can be given to large populations with and without symptoms, the actual number of infected people will be known.  See yesterdays blog about testing for an over view of antibody testing.    Covid 19 Tests: What we can and can’t say.

In the mean time, Stanford researchers reported doing this type of antibody screening in Santa Clara County.  In this test, the researchers chose 3300 Santa Clara residents that were representative of the residents of Santa Clara County.  At the time of this study, Santa Clara County had the largest number of confirmedcases of any county in Northern California (1,094). The county also had several of the earliest known cases of COVID-19 in the state -including one of the first presumed cases of community-acquired disease -making it an especially appropriate location to test a population-level sample for the presence of active and past infections.  Today (April 20, 2020) there have been 1820 Covid 19 cases reported in Santa Clara County.

They measured both the IgG and IgM antibodies which are indications of recent and earlier infections.  The prevalence (percentage of people who have the disease at the time of the study) was between 2.5 and 4.2%.  The population of Santa Clara County is 1.93 million people, so this would indicate 48250 to 81060 people were actually infected.  As there have been 73 deaths attributed to Covid 19, this new value makes the fatality rate .097 to .10 % – the same as for the seasonal flu.

Of course, it remains to be seen what the data looks like in a bigger area and across the US.  However, it is interesting to note that similar antibody studies in Europe (where asymptomatic people are tested as well) have shown the prevalence of Covid 19 in Robbio, Italy to be 10% and 14% in Gangelt, Italy.  This would suggest that the fatality rates in these area are also in the range of the seasonal flu.

Again, this is NOT downplaying the devastation of Covid 19 – but rather that although Covid 19 appears to be more contagious, the actual fatality rate hopefully, may not be very different from the seasonal flu.  There are currently 41575 deaths due to Covid 19 (18776 (45%) in NY alone).

It is reminded that in 2017 flu season, 44 million people got the flu in the US with 61000 deaths (1.3% fatality) so comparing Covid 19 to the flu does not diminish the terrible losses caused by the disease. 

One of the benefits of increased antibody testing that actual numbers for the disease will become known and it will better inform us on how to make health care decisions.  The good news continues to be that we, as a country, seems to have ‘stabilized’ as hospitilization rates have decreased everywhere.  NY continues to drive our nation wide statistics.  Your local area is likely not to be as bad as NY.  This and other data should guide the reopening of business and life.

You can read theentire Stanford paper at:  https://www.medrxiv.org/content/10.1101/2020.04.14.20062463v1.full.pdf


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