How long will the vaccine protection last?
This one can’t be answered with total confidence by any other way than just waiting and watching. But we will be able to give a meaningful answer well before that, fortunately. Here, just out in the last couple of days, is the most long-term and comprehensive look at the duration of immunity in recovered coronavirus patients. In fact, it appears to be the largest and most detailed study of post-viral-infection immunity in the entire medical literature (!) It’s from a multi-center team at the La Jolla Institute for Immunology, UCSD, and Mt. Sinai, and it looks at 185 patients who had a range of infection experiences, from asymptomatic to severe. 38 of the subjects provided longitudinal blood samples across six months.
We’ve already seen from the convalescent plasma comparison samples in the various vaccine Phase I trials that the antibody response to coronavirus infection can be quite variable, and that was the case in this study as well. That gives you wide error bars when you try. to calculate half-lives, and it’s not even clear what kind of decay curve the antibody levels will best fit to (it might well be different in different patients). But one figure to take home is that 90% of the subjects were still seropositive for neutralizing antibodies at the 6 to 8 month time points. The authors point out that in primate studies, even low titers (>1:20) of such neutralizing antibodies were still largely protective, so if humans work similarly, that’s a good sign. An even better sign, though, are the numbers for memory B cells, which are the long-term antibody producers that help to provide immunological memory. B-cells specific to the Spike and to the nucleocapsid coronavirus proteins actually increased over a five-month period post-symptom-onset, thus with no apparent half-life at all. These had interesting variations in antibody type (by the end of the period, they were strongly IgG, the others having dropped off), but as the paper notes, we really don’t have many viral infection profiles in humans to compare these results to. B-cell memory overall, though, looks to be long-lasting, and is expected by these results to stretch into years. For what it’s worth, there are patients who survived the 1918 influenza pandemic who had B cells that still responded with fresh neutralizing antibodies after over 90 years, so they can be rather hardy.
What about the other immune (and immune memory) component, T cells? The news there is good as well. CD4+ and CD8+ memory T cells appear to have half-lives of at least five or six months in these patients, and helper T cells (crucial for those memory B cells to respond later on) were completely stable over the entire period studied. Again, there are very few viral infection studies to compare this one to, but these numbers look consistent with long-term protection via reactivated immune memory.
Looking over the whole set of patients, it was clear that the immune system’s famously individual character was on full display here. That heterogeneity could well be the reason that there are real cases of re-infection, although it still seems to be rare. Different components of the immune response (both in antibodies and T cells) varied widely among patients, and these differences only became more pronounced over time. Nevertheless, at the five-month time point in a measure of five components of immune response and memory, 96% of patients were still positive on at least three of them (the categories were IgG antibodies against the Spike receptor-binding domain (RBD), IgA antibodies against the same Spike RBD, memory B cells aimed at the RBD, total SARS-CoV-2-specific CD8+ T cells, and total SARS-CoV-2-specific CD4+ T cells).
Bottom line: Taken together, this study, several others over the past few months, and this recent work all paint a consistent picture of a strong, normal, lasting immune response in the great majority of patients. Add in the results we’re seeing from the two vaccines that have reported interim data so far, and I think that the prospects for lasting immunity from vaccination are also very good. Remember, the early vaccine data suggested antibody responses at least as strong as those found in naturally infected cases. There seems (so far) every reason to think that vaccine-based immunity will be as good or better than that conferred by actual coronavirus infection. I very much look forward to more data to shore up this conclusion, but that’s how it looks to me at the moment.