Declining Vaccine Effectiveness


In a recent post I recommended a booster vaccination shot. I came to this conclusion based on reports from the Mayo Clinic that showed reduced effectiveness of both the Pfizer and Moderna vaccines, especially the Pfizer. Many friends who have been vaccinated also reported testing positive recently, although their illnesses were mild.

Now comes additional support for booster shots in a new study published in The New England Journal of Medicine (NEJM). This study is particularly important because it was done in healthcare workers at The University of California San Diego (UCSD). This well-documented study should convince everyone of the need for additional vaccinations.

UCSD began vaccinating healthcare workers in December 2020 with the mRNA vaccines. By March, 76% had been vaccinated and by mid-July 87%. Infections declined rapidly beginning in February, 2021. Between March and June, fewer than 30 healthcare workers tested positive each month. But this all changed in mid-June.

Coincident with the rise of the Delta variant and the end of the California mask mandate, the number of cases began rising. By the end of July, the Delta variant accounted for 95% of the UCSD isolates. The number of infections continued to rise, including cases among fully vaccinated persons. These changes are reflected in Figure 1 below.

Fortunately, UCSD has a low threshold for Covid testing, which is triggered by the presence of at least one symptom during daily screening or by an identified exposure, regardless of vaccination status. From March 1 to July 31, 2021, a total of 227 UCSD healthcare workers tested positive for Covid-19. Of these 227, 130 of these (57.3%) were fully vaccinated. Of these 130, symptoms were present in 109 or 84%. Symptoms were present in 80 of the 90 workers who were unvaccinated or 89%. No deaths were reported in either group and only one unvaccinated person was hospitalized.

Vaccine effectiveness was calculated for each month. Vaccine effectiveness exceeded 90% from March through June but fell to 65.5% in July. These changes are reflected in Table 1.

These same vaccines had previously demonstrated effectiveness of 94% for the Moderna vaccine and 95% for the Pfizer vaccine.

The authors conclude “. . .our data suggest that vaccine effectiveness against any symptomatic disease is considerably lower against the delta variant and may wane over time since vaccination. The dramatic change in vaccine effectiveness from June to July is likely to be due to both the emergence of the delta variant and waning immunity over time, compounded bby the end of masking requirements in California and the resulting greater risk of exposure in the community.”

This should be sufficient proof of the wisdom of getting a vaccine booster shot if your last vaccination shot was greater than six months ago. It is clear from this data that vaccine efficacy declined about six months after initial vaccination and coincident with the rise of the Delta variant. The annual season for influenza vaccinations is upon us and this would be a good time to get both. The CDC has already given us their blessing to get both the same day.