CDC - Studies on COVID Seroprevalence Indicate Higher Rates of Past Infection

Two reports were posted by CDC MMWR that look at how many people may have actually been infected with the virus versus cases that are actually reported.  By looking at seroprevalence, whether people had antibodies to the virus, both studies showed that at the time of sampling, the actual level or percent of people infected is far higher than what what had actually been reported.   The Indiana study showed that the actual rate may be close to 10 times higher than reported.

• These studies were conducted in late April / early May, and so, if this same rate of non-reported cases of infection is similar, than we do have a much larger population that has encountered the virus.
• These studies show that the mortality rate is lower when looking at the total number of actual cases versus reported.
• They do not know for sure that the presence of antibodies means the people are actually immune.
• The issue of asymptomatic people spreading the virus is a concern, so precautions like mask wearing and social distancing still must be in place.  Protection of high risk populations is also a must.

It is unfortunate that we do not have more information regarding actual rate of infection.  Knowing this would allow better decision making.  This could impact school and business operations.

However, mitigation strategies are still important.  Even though a larger percentage of the population may be infected than we know, there is even a larger percentage that has not been infected.

At this point, reporting negative cases of current infection tells us little.  Not to say testing is not important, but reporting how many people tested negative is not as important as percent positives there are.  Throw in the length of time it is taking to get results back, in same cases 3 to 7 days...or even longer, then the benefit of testing drops, and the reporting becomes less meaningful.

Have you already been infected?

https://www.cdc.gov/mmwr/volumes/69/wr/mm6929e1.htm
Population Point Prevalence of SARS-CoV-2 Infection Based on a Statewide Random Sample — Indiana, April 25–29, 2020
Early Release / July 21, 2020 / 69

Summary
What is already known about this topic?
No state has conducted a random sample study to determine the population prevalence of SARS-CoV-2 infection at a given point in time.
What is added by this report?
In a random sample of Indiana residents aged ≥12 years, the estimated prevalence of current or previous SARS-CoV-2 infection in late April 2020 was 2.79%. Among persons with active infection, 44% reported no symptoms.
What are the implications for public health practice?
The number of reported cases represents an estimated one of 10 infections. Given that many persons in Indiana remain susceptible, adherence to evidence-based public health mitigation measures (e.g., social distancing, consistent and correct use of face coverings, and hand hygiene) is needed to reduce surge in hospitalizations and prevent morbidity and mortality from COVID-19.

• During April 25–29, 2020, Indiana conducted statewide random sample testing of persons aged ≥12 years to assess prevalence of active infection and presence of antibodies to SARS-CoV-2;
• The overall prevalence of current and previous infections of SARS-CoV-2 in Indiana was 2.79% (95% CI = 2.02–3.70). In the random sample, higher overall prevalences were observed among Hispanics and those who reported having a household contact who had previously been told by a health care provider that they had COVID-19. By late April, an estimated 187,802 Indiana residents were currently or previously infected with SARS-CoV-2 (9.6 times higher than the number of confirmed cases [17,792]) (3), and 1,099 residents died (infection-fatality ratio = 0.58%). The number of reported cases represents only a fraction of the estimated total number of infections.
• The results of this large statewide population prevalence study, in a state with a population of 6.73 million,§ indicate that an estimated 187,802 Indiana residents were infected with SARS-CoV-2 from the start of the pandemic through April 29, 2020, a population prevalence of 2.8%. The finding that more persons had samples that tested positive for SARS-CoV-2 by RT-PCR, indicating an active infection, than for SARS-CoV-2 antibodies suggests that Indiana was in the early stage of the pandemic when the study was conducted. In late April, a total of 17,792 COVID-19 cases had been confirmed using conventional testing strategies (3), and were reported in the state, including 1,099 COVID-19–associated deaths. Based on the estimated total number of infections, the estimated infection-fatality rate was 0.58%, or approximately six times the 0.1% mortality rate for influenza (5). This fatality rate is lower than the infection-fatality rate of 1.3 observed on a cruise ship (2) but consistent with an extrapolate.

https://www.cdc.gov/mmwr/volumes/69/wr/mm6929e2.htm
Estimated Community Seroprevalence of SARS-CoV-2 Antibodies — Two Georgia Counties, April 28–May 3, 2020
Early Release / July 21, 2020 / 69
Summary
What is already known about this topic?
SARS-CoV-2 infection in persons who are asymptomatic or not tested might not be recognized by case-based and syndromic surveillance; therefore, the population prevalence of past infection might be unknown.
What is added by this report?
A community seroprevalence survey, conducted in two counties in metropolitan Atlanta during April 28–May 3, using a two-stage cluster sampling design and serologic testing, estimated that 2.5% of the population had antibodies to SARS-CoV-2.
What are the implications for public health practice?
Serologic surveillance can complement case-based and syndromic surveillance. At the time of this survey, most of the two-county population had not been previously infected with SARS-CoV-2, highlighting the importance of continued mitigation measures to prevent infection, including social distancing, consistent and correct use of face coverings, and hand hygiene.

• In this metropolitan Atlanta survey, an estimated one half of seropositive persons recalled having had a COVID-19–compatible illness, approximately one third sought medical care for the illness, and even fewer had a test for SARS-CoV-2 infection. These findings highlight that many SARS-CoV-2 infections would have been missed by case-based surveillance, which requires receiving medical care in the health care system or a test for SARS-CoV-2, and by syndromic surveillance, which relies on symptomatic illness. As testing practices change during the course of the pandemic, this pattern, reflecting findings at the end of April, might also change.