AstraZeneca

by Gertrud U. Rey

Have you ever wondered if you can “mix and match” SARS-CoV-2 vaccines? For example, would it be ok to boost a first dose of the vaccine produced by AstraZeneca with a dose of the vaccine produced by Pfizer/BioNTech? The latest science shows that such a vaccine regimen actually induces a stronger immune response than a regimen consisting of two doses of the same vaccine.

The occasional incidence of thrombosis in people under the age of 60 after receiving an adenovirus-vectored vaccine like the ones made by AstraZeneca and Johnson & Johnson has prompted several European governments to recommend the use of these vaccines only in people over 60. Because many people under 60 had already been vaccinated with a first dose of the AstraZeneca vaccine and still needed a second dose, they had to decide whether they should continue their regimen with another dose of the same vaccine (i.e., a homologous regimen), or receive an mRNA vaccine instead (i.e., a heterologous regimen).

In an effort to evaluate the efficacy of a heterologous SARS-CoV-2 vaccine regimen, the authors of this Brief Communication engaged the participants of an ongoing clinical trial, which aims to monitor the immune responses to SARS-CoV-2 in health care professionals and individuals with potential exposure to SARS-CoV-2. All study participants had already received a first dose of the AstraZeneca vaccine (referred to as “ChAd” in the study) and were given the option of receiving a second dose of the same vaccine or a dose of Pfizer/BioNTech’s mRNA vaccine (referred to as “BNT”). Although both of these vaccines encode the gene for the full-length SARS-CoV-2 spike protein, it is housed slightly differently. In the ChAd vaccine, the spike protein is encoded in an adenovirus vector of chimpanzee origin, while in the BNT vaccine it is surrounded by a lipid nanoparticle.

Out of the 87 individuals who participated in the study, 32 chose a second dose of ChAd and 55 chose to be vaccinated with BNT instead. Participants who chose a homologous ChAd/ChAd regimen received their second dose of ChAd on day 73 after the initial dose and donated a blood sample for analysis 16 days later. Participants who chose a heterologous ChAd/BNT regimen received their dose of BNT 74 days after their initial ChAd dose and donated a blood sample 17 days later. All results obtained from the analyses of the blood samples from these two groups were compared to results obtained from a group of 46 health care professionals who had been vaccinated with two doses of BNT (i.e., a homologous BNT/BNT regimen).

Briefly, the findings were as follows. Relative to the antibody levels induced by the first dose, homologous boosting with ChAd led to a 2.9-fold increase in IgG antibodies against the SARS-CoV-2 spike protein. IgG antibodies are mostly present in the blood and provide the majority of antibody-based immunity against invading pathogens. In contrast, heterologous boosting with BNT led to an 11.5-fold increase in anti-spike IgG, within the range observed in BNT/BNT-vaccinated individuals. A similar pattern was observed with anti-spike IgA antibodies, which are predominantly found in mucus membranes and their fluids. Boosting with BNT induced significantly higher increases in anti-spike IgA than did boosting with ChAd, suggesting that a heterologous boosting regimen induces better antibody responses. Interestingly, although the booster immunization induced an increase in neutralizing (i.e., virus-inactivating) antibodies in both vaccination groups, only heterologous ChAd/BNT vaccination induced antibodies that neutralized all three tested variants (Alpha, Beta, and Gamma), similar to what was observed in BNT/BNT vaccine recipients.

The authors also analyzed the effect of the two different boosting regimens on spike-specific memory B cells, which circulate quiescently in the blood stream and can quickly produce spike-specific antibodies upon a subsequent exposure to SARS-CoV-2. All vaccinees from both the ChAd/ChAd and ChAd/BNT groups produced increased levels of spike-specific memory B cells after receiving their booster shot, with no significant differences observed between the two groups. These results emphasize the importance of booster vaccination for full protection from SARS-CoV-2 infection.

ChAd/BNT recipients also had significantly higher levels of spike-specific CD4⁺– and CD8⁺ T cells compared to ChAd/ChAd recipients. CD4⁺T cells are a central element of the adaptive immune response, because they activate both antibody-secreting B cells and CD8⁺ T cells that kill infected target cells. Compared to the ChAd/ChAd regimen, ChAd/BNT vaccination also induced significantly increased levels of T cells that produce spike-specific interferon gamma – a cytokine that inhibits viral replication and activates macrophages, which engulf and digest pathogens. Overall, these results suggest that a heterologous ChAd/BNT regimen induces significantly stronger immune responses than a homologous ChAd/ChAd regimen.

The study did have several limitations. First, it was not a randomized, placebo-controlled trial where each participant was randomly assigned to an experimental group or a control group. Randomizing trial participants eliminates unwanted effects that have nothing to do with the variables being analyzed, so that the only expected differences between the experimental and control groups are the outcome variable studied (efficacy in this case). Subjects in a control group receive a placebo – a substance that has no therapeutic effect, so that one can be sure that the effects observed in the experimental group are real and result from the experimental drug. Second, the authors were unable to test the antibodies for their ability to neutralize the Delta variant, which has recently become the predominantly circulating variant in many parts of the world. Third, the study mostly included young and healthy people, making it difficult to extrapolate the data to other specific patient groups outside of this category. Fourth, the data only show the results of assays performed in vitro, which may not necessarily manifest a clinical significance. Extended studies aimed at determining the practical importance of the observed immune responses are needed to validate the relevance of these responses.

Despite its limitations, the study provides information that could have some valuable practical implications. Most of the currently approved SARS-CoV-2 vaccine regimens involve two doses of the same vaccine. However, access to two doses of the same vaccine may be limited or absent under some circumstances, thus necessitating the use of a different type of vaccine to boost the first dose.

[For a more in-depth discussion of this study, I recommend TWiV 782.]

On July 20, 2020, Oxford University’s Jenner Institute and the pharmaceutical company AstraZeneca reported preliminary results from phase I/II clinical trials assessing the safety and efficacy of a vaccine candidate against SARS-CoV-2.

The vaccine candidate, named AZD1222 (referred to in the publication as ChAdOx1 nCoV-19), consists of an adenovirus vector with an inserted gene that encodes the full-length SARS-CoV-2 spike protein. The notion of using a virus as a vector to deliver vaccines to humans is based on the ability of viruses to enter cells by attaching to host cell receptors and releasing their genome into the cell. Upon injection into a vaccine recipient, the vaccine vector should enter cells and serve as a code for host proteins to synthesize the SARS-CoV-2 spike protein from the inserted gene. Ideally, the spike protein will then act as an antigen to prime the immune system to recognize SARS-CoV-2 if it infects the body at a later time. As described in last week’s summary of the Moderna vaccine trial, the spike protein has been the primary antigenic choice for several SARS-CoV-2 vaccine candidates, because it mediates both binding of the virus to the ACE2 host cell receptor and fusion of the viral particle with the host cell membrane.

Adenoviruses are particularly suitable as vectors for delivering foreign genes into cells because they have a double-stranded DNA genome that can accommodate large segments of foreign DNA, and because they infect most cell types without integrating into the host genome. However, due to the prevalence of adenovirus infections in humans, most people have adenovirus-specific antibodies that could bind and neutralize these vectors, thus rendering them less effective at stimulating antibodies to the foreign gene product. To circumvent this problem, the Oxford/AstraZeneca investigators used an adenovirus of chimpanzee origin that does not normally infect humans. Thus recipients would not likely have pre-existing antibodies to the adenovirus vector itself. They also further optimized the virus by deleting two genes. Deletion of the first gene – which regulates viral replication – ensures that the virus cannot cause an infection in human cells. Deletion of the second gene creates more space inside the vector to allow for insertion of the gene coding for the SARS-CoV-2 spike protein.

The Oxford/AstraZeneca combined phase I/II clinical trial enrolled 1,077 healthy adult volunteers aged 18-55 who were randomly assigned to receive AZD1222 or a control vaccine. The control vaccine was a licensed vaccine against meningitis and is not viral-vector based. The trial was single-blinded, meaning that only investigators knew whether any particular subject received AZD1222 or the control vaccine, while the subject did not know. This is in contrast to double-blinded studies, where neither the investigators nor the subjects know who is receiving a particular treatment. The investigators chose the meningitis vaccine as a control to ensure that subjects remained blinded to which treatment they were receiving, because, like some vaccines, the meningitis vaccine causes a slight reaction. Use of a saline control would not have induced a reaction, causing subjects to possibly suspect that they received a placebo, which could create a subjective bias and affect experimental outcomes. The AZD1222 vaccine was administered as an intramuscular injection at a single concentration of 50 billion viral particles.

Volunteers were assigned to one of the following groups:

Group 1 – 88 subjects who received a single dose of AZD1222 or control vaccine were assessed for both side effects and vaccine immunogenicity;

Group 2 – 412 subjects who received a single dose of AZD1222 or control vaccine were assessed for both antibody and T cell immunity;

Group 3 – 10 subjects who received two doses of AZD1222 at a 28-day interval were assessed for both side effects and vaccine immunogenicity; and

Group 4 – 567 subjects who received a single dose of AZD1222 or control vaccine were only assessed for antibody immunity.

The basic findings were:

Side effects were mild to moderate, mostly consisting of pain and tenderness at the site of injection. Participants had the option of taking acetaminophen (paracetamol) prior to vaccination, which prevented injection site pain and tenderness in at least half of those who took it, and had no effect on vaccine immunogenicity.
All recipients of a single dose of AZD1222 (groups 1, 2, and 4) produced high levels of spike protein-specific total binding antibodies that were sustained to day 56 post-vaccination, and most subjects in these groups also produced neutralizing antibodies.
The second dose of AZD1222 boosted the levels of existing total binding antibodies and induced neutralizing antibodies in all subjects in group 3.
A single dose of AZD1222 induced high levels of spike protein-specific T cell responses in all subjects through day 56 post-vaccination. The second dose given to group 3 subjects did not boost these responses.

In general, the results of the trial are reassuring; AZD1222 seems to activate both arms of the adaptive immune response by inducing both neutralizing antibody and T cell responses specific to the SARS-CoV-2 spike protein. The study also involved a good number of subjects, a factor that is crucial for determining whether results are statistically significant for a phase I/II trial. Due to ethical implications, the efficacy of the vaccine cannot be tested by intentionally infecting (challenging) immunized subjects with SARS-CoV-2. However, results from such a challenge experiment previously done in rhesus macaques suggest that AZD1222 protected the animals against SARS-CoV-2 infection. Immunized macaques infected with SARS-CoV-2 had no signs of virus replication in the lungs, significantly lower levels of respiratory disease, and no lung damage compared to control animals. Even though macaques are not people, their immune responses often parallel those of humans and can provide important insights into human immunity.

Nevertheless, the trial also had several limitations. The vaccine was not tested in subjects over the age of 55, a group who often mount a weaker immune response and are at higher risk for severe COVID-19. The authors acknowledge the importance of a SARS-CoV-2 vaccine for this age group and note that the adenovirus vector used to make AZD1222 was previously shown to be immunogenic in individuals aged 50-78 when used to deliver an influenza virus vaccine. The majority of the volunteers were white, and it is well known that ethnic groups are disproportionately affected by COVID-19. The follow-up period was short, so we don’t know how long the observed immune responses will last and if there are any long-term side effects. It is also unclear why the study was only blinded to vaccine recipients, since double-blinded studies lead to more authentic conclusions because they reduce researcher bias.

The results obtained so far regarding the safety and efficacy of AZD1222 are only preliminary. Phase III trials aimed at assessing the vaccine’s efficacy in ethnically diverse populations as well as in older age groups with comorbidities are currently ongoing in Brazil, South Africa, and the UK, and will hopefully yield more conclusive results.

[The Oxford/AstraZeneca phase I/II clinical trial was also discussed on TWiV 644.]

Heterologous Vaccine Regimens Might be Better

Preliminary phase I/II results of ChAdOx1 SARS-CoV-2 vaccine