Delta Variant is More Contagious And Appears to be More Virulent

Many studies have now shown that the Delta variant is about 2.3x more contagious than the original SARS-Cov-2.  Some people suspect that it could also be more deadly but there is no clear evidence for this.  Much anecdotal evidence points to there being more young adults and kids being seriously ill and hospitalized during the latest Delta wave.  But are there more people hospitalized and dying from Delta than before? 

Are cases resulting in more hospitalizations?  For the period before 6/1/21, 8.1% of all COVID-19 cases in Florida (FL) required hospitalization (CHR = Hospitalizations/Cases).  For the last 100 days when Delta began to dominate, 9.3% of all cases resulted in hospitalizations.  You may have noticed in the graph below that this trend that had brought the hospitalization rate briefly over 10% (note that the hospitalization rates are scaled at 1/10th the scale of cases) has reversed over the last 3 weeks. 

The reason for this is that since schools reopened more pediatric cases have been identified and even though they do get hospitalized they do so at a much lower rate than older adults.  This tends to drag the overall CHR lower.  We used to be able to calculate CHR by age but since FL went to weekly reporting with far less detail it has been difficult to get CHR by age.  An alternative is to measure total new hospitalizations/capita/year versus age.  Delta produces higher hospitalization rates because it spreads faster and when it does infect, it produces more hospitalizations for most groups biased toward the younger (red curve below). Furthermore if we adjust for vaccination status (~10% for the period covered by the blue curve and 50% for the period covered by the red curve) the danger of the Delta variant is even more pronounced. 

Another way to look at the severity issue is to calculate the lagged Case Fatality Rate [CFR = deaths/(cases 3 weeks prior)].  The curve below shows a marked increase in CFR to near 2.0% from a 2021 average of 1.3% beginning in June with an apparent drop off in August.  Recently it has moved back up to near 1.6%.  This is surprising given that the median age of confirmed cases has dropped due to kids being infected from schools reopening dangerously.  Moreover, we would expect CFR to drop over the time as more people get vaccinated since the CFR for vaccinated people are expected to be lower and even lower still for those boosted (this is what is observed in the UK and Israel). 

The age effect for CFR is very strong with older people much more susceptible to deaths than younger folks.  This can be seen in the graph below where CFR by age for the life of the pandemic in FL is shown in blue.  Similar data for all deaths reported during the 5 week period from 8/5-9/9 is shown in red.  You can see that the red curve is higher for all age groups under 65 years old.  This seems to suggest that the Delta variant is more deadly.  The 65+ age group is far better vaccinated (87%) than any others in FL so a higher percentage of these deaths are probably breakthroughs with lower CFR than for unvaccinated of the same age. 

Given that all this data suggest that the Delta variant is more contagious AND more virulent, especially for innocent kids, we need more accurate, timely, and consistent information from all the states, the CDC, and HHS – not less.  It should be criminal to withhold such data from the public, the President, and the scientists who are desperately trying to understand SARS-CoV-2 better so we can control the pandemic.