Some Key Data on the Delta Variant from the CDC.
As usual, a disclaimer. I am just a reporter, not an infectious disease doctor or an epidemiologist. The CDC slides are from a talk to the CDC revealed by the Washington Post, and now public.
First, there is the importance of getting the one of the Covid-19 vaccines, especially considering that there are three or four weeks between the first and second doses of the most effective and prevalent ones, Pfizer and Moderna, and then two more weeks for them to develop full defenses.
The short blue columns are the weekly incidents of Covid-19 for the fully vaccinated per 100,000. The very tall green columns are the weekly incidents among the unvaccinated. The incidence of getting infected is 8 times greater among the unvaccinated. The incidence of hospitalization or death is 25 times greater among the unvaccinated.
The transmission factor R or number of people infected by each sick person is on the x-axis. The fatality rate in percent is on the y-axis. R for the Delta variant now ranges from 5 to 9 or greater on the linear scale. If this is a bell shaped probability for R it centers around 7. The initial or ancestral Covid-19 was mostly 1.5-3 on the graph, but in practice closer to just 1. R depends on the area and the extent of vaccination, masking and distancing. Whereas the ancestral strain was as transmissible as the common cold, the Delta variant is closer to chicken pox, who’s vaccine did not come out until 1995. The lethality in percent of the Delta variant, as high as 1.5%, is shown with only a slightly higher range than the ancestral strain of 1%.
Probability of incidence of 50 cases per 100,000 per week with only 60% vaccination coverage (vehicle dashed line) will be almost 100%, in the lower left red boxed graph. The assumptions made are given on the left hand column, with only masking of the source person about 50% effective, and of the target person about 25% effective. The top brown area is with no masking, the bottom light gray is with all masking, and the middle yellowish area is with only the unvaccinated masking. First of all, we haven’t quite reached 50% fully vaccinated yet. Then, R of 5 is just the very start of the R range of the Delta variant. With the current CDC policy of only the unvaccinated masked, the yellow area, it is almost 100% certain that we will have 50 cases per 100,000 a week. For our population of 330 million, that would be 165,000 cases nationally a week. At 1.5% lethality, that would be 2,500 Delta variant deaths per week, or almost a 9/11 event every week. In a year, that would be 130,000 deaths. With everybody masking, the light grey area on the bottom, the probability would range from 0 to 50%.
The next slide in the sequence is the same as that above, but with the following across it in Red letters in a Red box: Given higher transmissibility and current vaccine coverage, universal masking is essential to reduce transmission of the Delta variant.
Some other data from the CDC talk:
The Viral Load of Breakthrough cases from the Delta variant is 10 times as much as with unvaccinated cases with the Alpha (UK) or other variants. So breakthrough cases have to be considered as infectious as those on unvaccinated people.
The Delta infections last 18 days, rather than the 13 days of ancestral cases, by testing viral loads.
Compared to the Alpha or ancestral variant the Delta infections are more serious, with twice the odds of hospitalization, 4-5 times that of being in an ICU, and death, 2-5 times, in different countries.
The talk was given by Dr. Meredith McMorrow, Master of Public Health
Co-lead, Vaccine Effectiveness Team, Representing EPI Task Force, on July 29, 2021.
