The Delta variant of COVID-19 is a very contagious and virulent beast.  When better data is required to understand this rapidly evolving virus CDC tracking of COVID-19 cases has gotten worse.  For breakthrough cases due to the Delta variant, only the bare minimum is available.  Nevertheless, they do track total cumulative weekly hospitalizations and deaths in the US (with many caveats).  These data allow us to track incremental weekly data by subtracting the previous cumulative total from the current week’s cumulative total.  The trend is disturbing.

Week EndingCum BK HospitalCum BK Deaths7-day BK Hosp7-day BK Deaths7-day total Hospital7-day total Deaths% of hosp% of deaths
8/2  7,101  1,507    862     244  10,000    2,868   8.5%    8.5%
7/26  6,239  1,263    325     122    6,000    2,133   5.4%    5.7%
7/19  5,914  1,141    725       78    9,000    1,978   8.1%    3.9%
7/12  5,189  1,063      
7/19-8/2   1,912     444   25,000    6,979   7.6%    6.4%

Source: CDC (

Cumulative breakthrough hospitalizations (column 2 in Table above) and cumulative breakthrough deaths (column 3) in the table above are not very useful for analysis since they cover a long period of time when there were only a few fully vaccinated Americans and few Delta variant cases.  For the original and early variants, the mRNA vaccines were extremely effective against infections and near 95% effective against hospitalizations and deaths.  But the Delta variant is much more contagious and possibly more virulent.  Given this uncertainty, it is very important to track how the Delta variant behaves against vaccinated and unvaccinated Americans.  The most recent CDC data releases allow us to do this for the total US.  Columns 4 and 5 give us the 7-day totals for breakthrough hospitalizations and deaths, respectively.  These weekly totals were small and formed a small percentage of the total US numbers until 5 weeks ago when Delta cases started to soar in the US.  Overall breakthrough hospitalizations and deaths began to soar 4 weeks ago.  Over the 3-week period from 7/12 to 8/2, 6.4% of all deaths were breakthroughs (column 9) and an estimated 7.6% of all hospitalizations were breakthroughs (column 8)in the US.  This is happening at a time when about half the US (49.6% as of 8/2) was fully vaccinated and about 83% of all COVID-19 cases were Delta (as of 8/2).  If the mRNA vaccine is 90% effective against death and if the same number of unvaccinated and vaccinated Americans were exposed to SAR-CoV-2 Delta over this time period we would expect only 5% of all deaths to be breakthroughs.  The fact that it is now above 5% is quite troubling.  All else being equal – a necessary assumption in the absence of disaggregated data – 8.5% breakthrough deaths last week implies vaccine effectiveness of 83% against COVID-19 deaths. One factor that may make this ratio high is whether other mitigation factors were employed by the unvaccinated such as consistent mask-wearing or social distancing that lowered the infection rate for this group.  If masks turn out to offer better protection than the vaccine against Delta infection then more unmasked vaccinated people would get infected increasing breakthrough deaths.

Furthermore, the trend has been worsening.  As Delta cases approach 100% of all cases we expect these breakthrough deaths to increase. Of course, as more Americans get vaccinated, the percentage will increase even further toward 100%.  Currently increasing percentages might mean that the protectiveness of the full vaccine may be losing efficacy.  Both Pfizer and Moderna have sought authorization to administer booster shots but the FDA and the CDC are balking, possibly for political and messaging reasons.  This is absolutely unacceptable, particularly for the immunocompromised.  Israel has now authorized booster shots, and they track variants and breakthroughs better than the CDC.  The FDA should also give full approval for the vaccines instead of dragging its feet.  The CDC must begin to track breakthrough cases with more timeliness and more detailed segmentation.  They should view this outbreak as a new spike that requires thorough study and vigilance – testing, tracking, and applying the full arsenal of mitigation measures. Loosening the mask guidance on May 15th was dangerous.   Declaring victory against COVID-19 on July 4th was premature.  Calling this a pandemic of the unvaccinated is an oversimplification that leads to incorrect behavior.

Every American must mask up to reduce infections, vax up to save lives.