Well, it’s finally here – eight weeks to the day after press-releasing some top line results, the full paper is out on the Moderna mRNA vaccine candidate’s Phase I trial. I’m very glad to see it – it’s going to be very important for the full data sets on all the vaccine candidates to be made public.
So how’s it look? As we found out back in May, we’re looking at three groups of 15 volunteers each, 18 to 55 years old, getting 25 µg, 100 µg, or 250 µg of mRBA-1273 in two doses 28 days apart. The vaccine itself is an RNA sequence for a trimer form of the S (Spike) protein of the coronavirus (similar to the Pfizer/BioNTech mRNA vaccine in that way). It comes with a transmembrane anchor and the S1-S2 cleavage site between the subunits still intact, stabilized in the “prefusion” conformation that it will present in the wild-type virus before it infects cells. That stabilization is through the substitution of two residues at the top of the S2 subunit with proline residues, the “S 2P” form, and the same trick has been used to stabilize other surface proteins of other viruses entirely. (For those who aren’t into protein engineering, proline is unique among amino acid residues in forcing a much more limited conformation in the protein chain, particularly when you have two of them back-to-back). It’s in a lipid nanoparticle (LNP) formulation
As mentioned before, all patients seroconverted within 15 days of the first dose. The antibody titers generated were dose-dependent and were much higher (several-fold) after the second round of injections. Adverse events (fever, chills, pain at the injection site) were definitely more common after the second injection, too, which is just what you’d expect. With that in mind, it’s worth noting the design of the trial, a standard one that’s quite sensible when you’re stepping in to tweak the immune system. The dosing started off with four “sentinel” patients at the lowest dose, followed by four in the middle dose. After those showed nothing serious, both of those groups were then fully enrolled. After Day 8 of the full dosing, four sentinel patients were injected in the highest-dose group, and after them, the rest of that group were enrolled. For the wrong way to try out a new immunology approach, see here.
The patients were assayed for antibody levels (in a standard ELISA format), for neutralizing antibodies (by looking for inhibition in various cell-infection assays), and for T-cell levels. As you’d expect, none of the patients’ plasma showed the ability to neutralize the coronavirus before the trial dosing began. And neutralization was still low after the first injection, although the antibody titers had gone up. By Day 43, though (post-second injection), all participants were able to neutralize the effects of the virus in the cell-infection assays by at least 80%, with those responses also being dose-dependent. A comparison showed that this activity was the same or higher than that found with the plasma of convalescent patients (samples from 38 people, collected 23 to 60 days after onset of symptoms). But one thing that you do notice as you look over the data was that day 43 was the best – there was a further evaluation at day 57, and all three groups had gone down a bit in just those two weeks. You can see this happening in the pseudovirus neutralization assays in the paper’s Table 2 and in Figure S8. These patients are no doubt continuing to be monitored, and it is of great interest to see how their neutralizing antibody titers hold up.
That said, antibody levels are not the only thing that determines immunity. T cells are a big part of this story, although we don’t know all the details – you’ll generally hear a lot more about antibody titers because they’re a lot easier to measure, and to be fair they are often a good proxy for overall immunity. But not always. As for the T-cell data here, CD4+ cell responses were noted, but there was much weaker CD8+ activity (and that only after the second dose in the 100 µg group). Those CD4+ cells can be further differentiated into Th1 and Th2 cells, which each produce a different suite of cytokines. In this case, the vaccine seemed to mostly elicit Th1. The balance between those two types is a complex subject indeed (they have different modes of action and can influence each other’s activity as well), and that also goes for the balance between the CD4+ and CD8+ T cells in general.
I’m not enough of an immunology geek to be able to tell you what profile we would be looking for, and I don’t think we even quite know yet. My impression is that CD8+ cells are more well-established as being important in clearing viral infections (especially respiratory viruses), but the CD4+ ones (and the ratio of the two) are real players as well. As for the Th1 and Th2 subsets of those CD4+ cells, there’s evidence that the Th1 type are more powerful against viral pathogens, at least for some viruses. The general belief, in fact, has been that Th1 cells are more important in fighting intracellular pathogens in general, with Th2 cells going after extracellular parasites and the like, but (like everything else in immunology) that framework has only become more complicated as we learn more about it.
So from my bozo-immunology perspective, I think at first glance that I would rather see a more robust CD8+ response than what Moderna has shown here. Others seem to feel similarly. But that said, I don’t know what the convalescent patient T-cell situation is, either: what kind of response did these people have when they cleared the virus on their own? We don’t have the figures from the set of patients in this paper (they just took plasma to evaluate antibodies). But we know from a study of 10 infected patients with respiratory distress that those patients had a higher CD4+/CD8+ ratio, and that they had a higher Th1 response among the CD4+ cells. But you’d want to hear about the people who recovered smoothly as well as about the ones who ended up on respirators, wouldn’t you? The main thing I’ve been able to find on that is this paper, which also showed a shift towards CD4+ cells in pooled plasma from convalescent patients, and among those cells a very pronounced Th1-driven response (note: more on this one here, and in a separate blog post, coming shortly). So the Moderna data might well resemble the profile of recovered patients, which doesn’t sound so bad, although keep in mind that there might still be better ways to clear the virus than the response that we tend to get. We’re just going to have to see how things play out in Phase II/III, aren’t we? One also would like to see such profiles for the other vaccines in the race, and I assume that we will.
The comparisons are going to be pretty darn interesting. As you can see, Moderna’s candidate is absolutely going to need two injections (as did the Pfizer/BioNTech vaccine candidate), and the reaction to the second dose is pretty vigorous. Will that cause trouble in moving into a larger and more diverse patient cohort? The Moderna neutralizing antibody response seems broadly similar to the Pfizer study, but we don’t have any T-cell profiling from Pfizer yet, so it’s impossible to make any comparisons in that department. The Pfizer/BioNTech adverse event profiling looked a bit better – is that going to be a distinguishing characteristic as the various vaccines go on? Will there be (can there be?) a single-dose vaccine from someone, which would make life and logistics much easier? What will the differences be in the strength of real-world protection against infection, and in its duration? I have no earthly idea, and neither does anyone else: that’s why everyone is charging into the later clinical trial phases.
