Asymptomatic infection rates

[DSeid]

A few bits to share that I think justify updating this thread.

1. From BMJ Thorax another cruise ship study.

We describe what we believe is the first instance of complete COVID-19 testing of all passengers and crew on an isolated cruise ship during the current COVID-19 pandemic. Of the 217 passengers and crew on board, 128 tested positive for COVID-19 on reverse transcription–PCR (59%). Of the COVID-19-positive patients, 19% (24) were symptomatic; 6.2% (8) required medical evacuation; 3.1% (4) were intubated and ventilated; and the mortality was 0.8% (1). The majority of COVID-19-positive patients were asymptomatic (81%, 104 patients).

Bolding mine.

These do seem to be truly asymptomatic rather than presymptomatic individuals, but I am not sure if that can be stated with complete confidence. Also unclear the age range of those on the ship … my WAG is that a cruise of the Antarctic Peninsula following “a route similar to that taken by the British explorer, Ernest Shackleton, in 1915–1917” might have a different demographic than a Carnival Cruise. Also noted that antibody studies done during an acute phase were falsely negative but follow up testing in not reported.
2) Wuhan. Population 11M. Workers required to be tested before being able to return to work. Very useful as this study was done not during a rapid rate of rise. Studies done while a population is on the curve are problematic as antibody positivity lags infection by 2 weeks or more.

50,008 confirmed cases and 2,579 deaths were from Wuhan … people have been allowed to resume working since April 8 if meeting a set of COVID-19-associated tests including SARS-CoV-2 nucleic acid test (NAT) of nasopharyngeal swabs, chest CT scan or a SARS-CoV-2-specific serological test. … there were 98 (98/1,021, 9.60%) from resuming group that were IgG positive and IgM and NAT negative. None of these people had a history of COVID-19, or suspect symptoms. These IgG+IgM-NAT- individuals with no history of COVID-19 probably suggested a recovered asymptomatic SARS-CoV-2 infection. …

If one assumes that the 9.6% with past asymptomatic rate that means the true number of infected individuals in Wuhan was about 1.1M and the death rate (using the number quoted in the article) was 2,579/1,100,000 = 0.2%. Double it for assumed under-reporting of deaths maybe? Asymptomatic infections are being reported as 21 times more than “confirmed cases.”
3) Spain study done 4/27 to 5/11. Information from reading the tables put into a Google translator and doing some simple calculations (40202 asymptomatic of which 2.5% were positive, so on). Positive IgG in: 1005 asymptomatic individuals, 46% of all who were positive; 568 with 1 to 2 symptoms, 26%; so on.

Very few had been confirmed by PCR infections (247 of the over 60K).

Overall

The estimated prevalence of IgG antibodies against SARS-Cov2 in Spain is 5.0% (95% CI: 4.7% -5.4%), being very similar in men and women (5.0% (95% CI: 4.6% -5.4%) versus 5.1% (95% CI: 4.7%).In relation to age, the prevalence is lower in babies, children and young people, remaining quite stable in older age groups.

The proportion of positives is higher in residents of large cities (> 100,000 inhabitants) 6.4% (CI95%: 5.8-7.1)

Using 5% with a deaths/million at the same time of about 530/million (0.05%) and IFR was running, very crudely about 1%.
4) England.(Page 22.) Current week is week 22.

… After making adjustments based on the latest information on the accuracy of the assay*, the overall adjusted prevalence in London increased from 1.3% in week 13 to 10.6% in weeks 15-16 and 14.8% in week 18. … Given that the antibody response takes at least two weeks to become detectable, those displaying a positive result in week 18 are likely to have become infected before mid-April …

Mid-April numbers for confirmed cases was about 1500/million (0.15%) and total deaths 2 weeks later (presumptively from infections up to that date) about 327/million (0.03%). Plugging in that’s 99 times more infected than confirmed cases and an IFR of 0.2%.
5) Sweden.

The Public Health Authority has launched a survey to measure and estimate how many in the community have had covid-19. Blood samples are collected from laboratories in clinical chemistry and clinical immunology in nine regions: Jämtland, Jönköping, Kalmar, Skåne, Stockholm, Uppsala, Västerbotten, Västra Götaland and Örebro.

The collection takes place during eight weeks in the spring of 2020. A total of 1,200 samples are collected each week for analysis of antibodies. Antibodies show that the immune system recognizes the SARS-CoV-2 virus.

The analyzes for week 18 (a total of 1,104 analyzed samples) show, as expected, the largest proportion of positive antibody tests in Stockholm. A total of 7.3 percent of the blood samples collected from people in Stockholm were positive in the antibody study, which can be compared with a total of 4.2 percent in Skåne and 3.7 percent in Västra Götaland.

The numbers reflect the state of the epidemic earlier in April, as it takes a few weeks for the body’s immune system to develop antibodies.

Regarding age differences, the results show that it was most common with antibodies to covid-19 among people between 20 and 64 years. In total, 6.7 percent of the samples in this group were positive, which can be compared with 4.7 percent in the age group 0-19 years and 2.7 percent in the age group 65-95 years.

Confirmed cases in Sweden in mid April about 1200/million (0.12%) and deaths 2 weeks later about 250/million (0.025%). About 59 times more with evidence of infection than confirmed infections and an implied IFR of 0.3%.
FWIW.