A Deep Dive On mRNA Vaccines

February 11, 2021

By Deborah Borfitz

February 11, 2021 | Messenger RNA (mRNA) vaccines were the focus of a symposium on COVID-19 vaccines held during last week’s COVID-19 and Cancer virtual meeting of the American Association for Cancer Research. To date, only two vaccines have received Emergency Use Authorization by the U.S. Food and Drug Administration and both are mRNA vaccines—one developed by Moderna and the other by Pfizer and BioNTech.

Speaking on development of the Moderna vaccine was Randall N. Hyer, M.D., Ph.D., the company’s senior vice president of global medical affairs. Michela Locci, Ph.D., assistant professor of microbiology at the University of Pennsylvania, presented on the ability of mRNA vaccines to elicit potent germinal center (GC) responses associated with neutralizing antibody generation. This was followed by a lively Q&A session moderated by Deepta Bhattacharya, Ph.D., a member of the cancer biology program at the University of Arizona Cancer Center.

Moderna’s COVID-19 vaccine development program utilizes an advanced mRNA technology platform also being used to develop other medicines and vaccines, says Hyer. The approach uses DNA to make mRNA that instructs cells to make a harmless piece of the spike protein found on the surface of the virus, triggering an immune response and the production of antibodies.

The vaccine does not alter DNA, he says. It also does not signal for nuclear access or reverse transcription. It contains no adjuvant at all, says Hyer. Rather, the vaccine appears to trigger the innate immune system.

At the lymph node, B cells (derived from the bone marrow) and T (thymus) cells interact with the spike protein and develop an adaptive immune response, he explains. Once the mRNA and protein it produces have done their job, they degrade after a day or two.

The vaccine has been produced in large quantity and formulated with lipid nanoparticles that are 100 nanometers in diameter, Hyer continues. In animal models, it demonstrated robust COVID-19 neutralizing antibody response and prevented the replication of the virus in the airways.

Adaptive Trial Design

Phase 1, 2, and 3 clinical studies overlapped to accelerate the traditional vaccine timeline, says Hyer. Convalescent serum from 41 individuals diagnosed with COVID-19 was used as a comparator, and participants ranged in age from 20 to 77 years. Antibody levels were detected and “declined very little,” and remained elevated for three months after a second booster shot.

The geometric mean titer and T-cell response “looked good” after the first dose, he says. “But it took a second dose to get above the levels of convalescent serum, so the booster is important.”

In trials, the second dose was administered about 28 days after the first but the “immunization window” extended out another 10 days. The maximum acceptable delay in receiving the second dose is unknown, says Hyer. Other multi-dose vaccines typically have a firm minimum interval, he adds, “but we’re in new territory here.”

In the phase 3 trial with 30,000 U.S. participants, randomized 1:1 to two shots of the vaccine or placebo, Moderna’s mRNA vaccine showed 94.1% efficacy overall with tight confidence intervals, says Hyer. Participants included individuals at risk of developing severe COVID-19 based on their age and comorbid conditions. Individuals 18-65 without comorbid conditions comprised 17% of the study population, people 18-65 with comorbid condition 17%, and those 65 and up (with and without comorbid conditions) 25%.

Vaccine efficacy remained “very high” for different racial groups, but case numbers were too small to calculate conclusively, Hyer adds. Importantly, there was no evidence of vaccine-associated enhanced respiratory disease. Cases of severe COVID-19 numbered 30 in the placebo group compared to zero for those in the vaccine group.

Mortality was lower among individuals in the placebo group than the population at large and, most notably, the elderly. The reason, Hyer says, is that study participants were “more sensitive to basic public health precautions, and we followed up on them.”

Systemic reactions to the vaccine—primarily pain at the injection site and flu-like symptoms—were higher after the second injection and each was more common among younger participants, reports Hyer. Some of those same ill effects were seen in the placebo group. Unsolicited adverse events were roughly the same in the placebo and vaccine groups.

Follow-up Studies

As recently announced by Moderna, in vitro neutralization studies indicate that the vaccine retains its neutralizing action against emerging variants, including those from South Africa (B.1.351), the U.K. (B.1.1.7), and Brazil (P.1), adds Hyer, but new studies are planned. It provides a similar level of immune protection against these new variants as it does the original Wuhan strain.

The company is also testing two different booster vaccines aimed at the concerning B.1.351 strain, Hyer says. Antibody levels produced by the Moderna vaccine, while above levels expected to be protective, were about six times lower with B.1.351 than prior variants.

If a variant emerges that becomes the dominant strain in circulation, using a mix of vaccines might be considered. “All options are on the table,” says Hyer. Unlike the 1918 pandemic, the second and faster moving wave of infection with SARS-CoV-2 is unlikely to end after a few months, even if COVID-19 and influenza are both mRNA viruses.

Moving forward, Moderna also plans to collect additional data specific to children, cancer patients, and pregnant women, all of whom were excluded from the initial round of clinical trials, he says. For the effects in cancer patients, Moderna is partnering on a study with the National Cancer Institute.

“We felt it was important to have as clean a look [as possible] at immunogenicity offered by the vaccine, says Hyer, noting that immunosuppressed people are typically not enrolled in vaccine trials. However, the Moderna trial included 176 people with HIV, among whom no unusual safety concerns were reported.

Memory B Cells

The race to come up with a safe and efficacious COVID-19 vaccine has had many contenders, including 173 candidates in preclinical studies and 63 vaccine candidates in clinical development, says Locci. Among them are RNA and DNA vaccines, recombinant protein vaccines, inactivated vaccines, and live attenuated vaccines.

The spike protein is the target of both the Moderna and Pfizer/BioNTech mRNA vaccines, and the receptor-binding domain (RBD) it contains is an ideal candidate for vaccine development, Locci says. While the immune cellular responses to mRNA vaccination is known to be good, she has endeavored to fill the information gap on the magnitude and quantity of memory B cells they generate.

As detailed in a recently published study in Immunity (DOI: 10.1016/j.immuni.2020.11.009), two SARS-CoV-2 mRNA vaccines—but not a recombinant protein vaccine formulated with the MF59-like adjuvant AddaVax—promote robust GC-derived immune responses in mice, she says. These include GC B and T follicular helper (Tfh) cell responses as well as long-lived plasma cells and memory B cells—all of which strongly correlated with neutralizing antibody production.

RBD-specific GC B cells peaked at seven days and fell a week later and induced the potent GC reaction. For at least 60 days post-immunization, immunoglobulin G (IgG) antibodies that are expressed by B cells were also significantly higher. The Tfh cells elevated by the mRNA vaccines also had stronger Th2 polarization, says Locci, which plays a key role in maintaining the delicate balance between antigen responsiveness and tolerance.

Locci speculates that memory B cells drive a secondary GC response with the second shot of the mRNA vaccines since they tend not to re-enter the germinal center upon boost.

Many Unknowns

The two hot topics discussed during the final Q&A session were transmissibility of the virus post-vaccination and how long to wait between the first and second doses.

To answer the transmission question, researchers will need to look for penetration of IgG nodes in the respiratory airways of larger animal models, says Locci. But she suspects this is the case, given the vaccine’s cellular presence is transient.

Up to 30% of infected individuals are asymptomatic, which would explain the rapid spread of COVID-19, Hyer says. With the SARS-CoV-1 in 2003, it was clear who was infected so they could be isolated. “We could shut it down after 840 cases. Now we see 840 cases before lunch, or more.”

An interesting finding from clinical trials for the Moderna vaccine was that 39 individuals in the placebo group and 15 in the vaccine group who were PCR negative for SARS-CoV-2 at baseline had nasopharyngeal swabs that were positive for the virus at the second dose but had no evidence of COVID-19 symptoms, says Hyer.

The implication is that even one dose of the vaccine could potentially reduce asymptomatic cases, which will need to be confirmed by larger follow-up studies. “If we can get a grip on any of the effects vaccination has on transmission of asymptomatic infection, that would be huge,” says Hyer.

As to the mechanism of early protection seen with the Moderna vaccine at around 14 days after the first dose, Locci points to antibodies as a contributing force. In mice as early as day seven, the vaccine produced robust (if short-lived) plasma cells, she says.

Hyer’s thought is that the mRNA technique has a “more pronounced effect upstream, turning the body into an in vivo vaccine antigen production [factory].” Some innate effects may simultaneously be taking place, he adds. “We don’t know.”

Interferon may be an “important player,” says Locci, indicating that this will be studied shortly.

“The mRNA vaccine may just be a potent inducer of innate immunity,” adds Hyer.

“The mRNA component is important, but the real star is lipid nanoparticles,” Locci says. A soon-to-publish study will show that they are “almost as good” as vaccine themselves at prompting an immune response.

In reply to questions regarding the interval between first and second vaccine doses of the Moderna vaccine, Hyer says it would be “folly” to assume knowledge from previous vaccines could be applied to this one. “It may very well be the second dose needs to occur within that [10-day] window” used in clinical trials.

“What’s striking is that two different efforts to develop prophylactic vaccination using a similar strategy but a different vaccine, different manufacturer, different lipids, and different populations had their point efficacy within 1% [of each other],” Hyer says.

In the absence of clinical data on immunocompromised patients on therapies for cancer, “it is up to individual doctors” to decide if the benefit of vaccination outweighs the risks, says Hyer. The only contraindication to the Moderna vaccine is anaphylaxis to the vaccine itself, he adds.

Even if patients’ antibody response is expected to be suboptimal, adds Locci, odds are good that an mRNA vaccine will produce CD4 (helper) and CD8 T (cytotoxic) cells. While both the Moderna and Pfizer vaccines trigger strong CD4 T-cell responses, Pfizer’s vaccine has been shown to elicit a more robust CD8 T-cell response, she says.

An advantage of vaccine delivery via lipid nanoparticles, relative to traditional approaches, may be that mRNA expresses the full-length spike protein and is a “natural process in the antigen-presenting cell,” says Hyer. The mRNA platform endeavors to “mimic natural infection processes, so I assume that has something to do with triggering innate response and other aspects of the immune system.”

Viral vector approaches face the possibility of anti-vector cellular immunity diminishing vaccine effectiveness, Hyer says, in reply to a question about their disadvantages.