We’ve noted a number of times that we are in a period of great uncertainty about the outlines of the COVID Pandemic. How vaccines are holding up over time and how well they protect against the Delta variant are both uncertain, with limited and contradictory data. Yesterday I noted new data out of Israel which appears to show a dramatic improvement in protection with a third shot. But that scale of improvement rests on other data out of Israel suggesting that effectiveness of the Pfizer vaccine had dropped significantly since early this year, especially against infection.
Yesterday the CDC published a new report that is at least directionally in line with Israeli data on vaccine effectiveness against infection, albeit showing a less pronounced drop.
The study published in the CDC’s weekly MMWR report is a continuation of an on-going surveillance based study of health care workers and other first responders. Participants in eight locations across six US states are tested weekly for COVID. So this significantly removes the wildcard of who chooses to get tested and whether mild infections are unreported because people don’t get tested.
Results of the study were first released for the December 2020-April 2021 period. The new report brings that up to date for the subsequent months over which the Delta variant became dominant in the US. The end date for the data is August 14th. From the first period to the second vaccine effectiveness against infection declined from 91% to 66%. In other words, a pretty significant drop.
This isn’t as big a drop as found in Israel. But it is comparable inasmuch as it shows a rather dramatic reduction. The authors are careful to note that the difference could be the prevalence of Delta or waning vaccine immunity or some mix of both.
One possibility that occurred to me is that this US study was of working age people. In Israel the most pronounced declines in vaccine effectiveness were among people in their late sixties and up. So possibly focusing on working age people only explains some of the difference between 39% effectiveness against infection in Israel and 66% in this study during the same general period.
Unfortunately the study does not include data on severity of disease, which is the most critical measure. Other recent studies show continued robust protection against severe disease even as vaccine effectiveness against all infection has decreased.