Questions that arise from yesterday’s CDC update on COVID-19

The CDC provided an update on the status of COVID-19 infection in the United States yesterday with a particular focus on the utility of the vaccines. It was a welcome message, including a formal analysis of breakthrough infections. The results showed that the vaccines continue to work even in the presence of the Delta variant virus, with vaccinated individuals 4.5 times less likely to be infected, 10 times less likely to be hospitalized, and 11 times less likely to die from coronavirus infection.

But behind these headlines, the update raised five questions that need to be answered.

Question 1: Are the vaccines less effective against the Delta variant virus? 

It does appear that there is a drop off in protection against infection from the Delta variant. This was most prominent in individuals over 65 years of age. Despite this, the RNA vaccines are still very robust in their ability to prevent hospitalization and death. Reflecting this, over 90% of the people currently hospitalized with COVID-19 in the United States are unvaccinated.

Question 2: What underlies the difference in protection in the elderly versus those below 65 years of age?

Older individuals now appear to have less effective immunity with the vaccine and are more likely to be infected and hospitalized than those under 65. This is different from the initial studies which were with the original virus. But it is not clear whether this is because of the increased transmission of the Delta variant or because of waning of immunity over time, which is more prominent in the elderly. In the latter regard, because the elderly were the first population immunized, they are the farthest out from vaccination. Therefore, some of the waning immunity may simply be due to the duration of time since these individuals were vaccinated.

Question 3: Is the Pfizer vaccine less effective in preventing infection efficacy than Moderna’s vaccine?

Moderna’s vaccine was still nearly 95% effective in preventing infection versus 80% for Pfizer (and 60% for Johnson & Johnson). The reasons for this difference aren’t entirely clear. First it could simply be dosing, since Moderna contains three times as much of the RNA material than Pfizer in each of their two shots. In contrast, Pfizer’s vaccine was available and used for a month before Moderna’s, therefore the time since immunization is again longer, particularly in the elderly. These two factors may together account for the difference in immunity currently seen against the Delta variant. So don’t jump to the conclusion you need a Moderna booster over Pfizer (see below).

Question 4: Does any of this data suggest that booster vaccines will improve immunity? 

It’s not clear from this data whether booster vaccines will be effective, especially in the elderly. If the decline in immune protection is simply due to a lower dose of the Pfizer vaccine or a longer time frame from vaccination, a booster shot should resolve differences between the vaccines and provide better protection. Data from Israel suggests this may be the case. In contrast, if the immune protection is not the problem, and people who are elderly or more debilitated with pre-existing conditions are more likely to get sick from the Delta variant, it is not clear that the booster vaccines will help that. Better studies after boosters will clarify this issue.

Question 5: Does this information provide reassurance that the vaccines can end the pandemic? The most heartening thing from this information is that the vaccines continue to be very effective against the spread of infection even with a more contagious virus. Most of the pandemic spread relates to the ease of infecting people. By reducing the likelihood of infection by almost fivefold, the vaccines can end the pandemic. Importantly all populations of individuals, including children, will need to be vaccinated to accomplish this. Hospitals and the healthcare systems, which in many states are being overwhelmed, also will be much better off as most of the individuals hospitalized are unvaccinated. Finally, the elderly will be better protected from infection by immunizing younger people who will be less likely to get infected and spread the infection. Together this suggests that finishing the job of immunizing everyone will essentially end the COVID-19 pandemic.