How Soon Is Another Booster Needed? Durability of Vaccine-Induced Immunity 

Kevin Kavanagh, MD

I am at high risk for severe COVID-19, 65 years or older, with additional health problems. I received my bivalent booster as soon as possible and am approaching my 6-month anniversary. I, thus, watched the CDC’s vaccine advisory committee (ACIP) with great interest regarding their recommendations for when to obtain my next booster.

Figure 1: Source: Christopher A. Taylor, PhD, CDC ACIP Presentations

Reuters reported that the CDC Committee concluded, “There is not sufficient evidence to recommend more than 1 COVID-19 booster shot a year for older people and those with weakened immune systems....” But the committee did voice some flexibility. It needs to be stressed that there was an absence or lack of data—rather than a presence of data—indicating the durability of vaccine immunity, justifying annual boosters.

There was no vote regarding the timing of booster doses, but there was nowhere near a unanimous consensus. Michael Hogue, PharmD, from the American Pharmacist Association (APhA) stated: “We want those clinicians to be able to make good decisions for the individual patient based upon their comfort and desire as long as we have safety in mind, and it is clear that we do have a very safe vaccine with our bivalent vaccine. So, I feel that flexibility needs to be put into this some way, with both older adults and people with immunocompromising conditions."
 
One committee member stated that those over the age of 65 should be allowed to discuss off-label use with their physician to receive the booster sooner. However, I did not hear calls for approving a more frequent administration schedule either. Almost all the discussion centered on mRNA vaccines, with little mention of Novavax or the urgent need for newer, more durable vaccines.

Figure 2: Source: Amadea Britton, MD, SM, CDC ACIP Presentations

 
Vaccine effectiveness is an important point. Although much of the younger population received a get-out-of-hospital free card for the latest variant, senior citizens were in its crosshairs. During the last 6 months (since May of 2022), those over the age of 75 had a higher rate of hospitalizations than in the Delta surge; those between the ages of 65 and 75 had approximately the same. Both age groups continue to be at high risk for death and disability. (Figure 1)

However, when I saw the presented vaccine effectiveness data, I felt foreboding. Data showed that for those 65 years of age and older, the monovalent vaccine’s (2 or more doses) effectiveness for preventing hospitalizations fell to 28% in less than a year. Results were worse, only 19% for those individuals under 65 years.

The young fared even worse with less durable immunity after monovalent vaccination. This paradoxical finding was possibly explained by the elderly leading a safer lifestyle and that immunity produced by previous infections may not have been as durable. Neither explanation bodes well for vaccine effectiveness lasting a year.

Data for the bivalent booster was also presented. Effectiveness was compared to that achieved by the monovalent vaccine (2 or more doses with an average interval since the last dose of 348 days or a little less than a year). Effectiveness for hospitalizations fell rapidly in a little over a month, from 52% at a median of 32 days to 31% at a median of 74 days (67-85) after the last dose. It should be noted that this effectiveness is on top of some residual immunity from the monovalent vaccine since the monovalent vaccine was used as the reference.

But let’s face it; the results are dismal, with little durability from an immunity boost from a bivalent booster after an individual has had a monovalent vaccine. The benefit from the bivalent booster rapidly diminishes over a few weeks. A 31% efficacy in preventing hospitalizations at a median of 74 (67 to 85) days after receiving the booster is poor and would be expected to be poorer in the elderly. (Figure 3)

Data regarding the prevention of emergency and urgent care visits for the monovalent vaccine were even less reassuring, with practically no benefit at1 year and rapid attenuation of the benefit provided by a bivalent booster, a 36% efficacy at a median time of 76 days (67-85) after receiving the booster.

Figure 3: Source: Amadea Britton, MD, SM, CDC ACIP Presentations

Is the bivalent booster worth taking? Yes, definitely. However, this differs from the booster or vaccine we need to navigate this pandemic. We need another warp-speed initiative for vaccine development—a vaccine that is more durable and can reduce spread. The risk of continuing to use a vaccine with reduced effectiveness is shown in the first data set, where the vaccine was seemingly less effective in the young. When vaccinated, many view themselves as invincible and can increase risky behavior far beyond the vaccine's benefits.

It is critical for the elderly to keep their immunity as high as possible. Monoclonal antibodies are no longer effective with the new variants, and far too many cannot receive Paxlovid because of drug interactions. Molnupiravir is often not prescribed since it works by creating viral mutations and has been implicated in speeding variant evolution.
 
At the conclusion of the CDC Committee meeting, my primary impression was that we senior citizens might be viewed as expendable. Far too few policymakers are concerned about our well-being and willing to make the hard decisions that must be made to assure our safety during this pandemic. After looking at the data, I will consult my physician about receiving a booster on an accelerated schedule, possibly at 6 months.