News & Views: Getting Familiar with COVID-19 Adenovirus-replication-deficient Vaccines

Although the U.S. is on the brink of having a new type of COVID-19 vaccine, the Johnson & Johnson (Janssen) adenovirus-replication-deficient COVID-19 vaccine will not be the first of its kind to be approved. Indeed, as of mid-February 2021, 11 COVID-19 vaccines are approved for use in different areas of the world — four are based on adenovirus platforms. In fact, adenovirus-based vaccines are currently being distributed in about 70 countries. So, let’s take a closer look at this type of vaccine that until now was widely studied to combat numerous pathogens, but only rarely approved. Specifically, before COVID-19 adenovirus-based vaccines were approved, only one other adenovirus-based vaccine was approved for use — a two-dose Ebola vaccine, of which the first dose is an adenovirus vector and the second is a recombinant vaccine. The Ebola vaccine was approved by the European Commission in July 2020. No adenovirus-based vaccines have previously been available in the U.S. Also, of interest, Johnson & Johnson’s adenovirus vaccine to prevent Ebola was approved down to 1 year of age.

COVID-19 adenovirus-based vaccines

The vaccine submitted for approval by Johnson & Johnson uses a human adenovirus to which most of the population does not have immunity, called adenovirus 26 (Ad26). As with the COVID-19 mRNA vaccines, this vaccine is directed against the SARS-CoV-2 spike protein. The Sputnik V vaccine, produced in Russia, uses both Ad26 (dose 1) and Ad5 (dose 2) vector viruses.

The AstraZeneca vaccine and an identical one being made in India, called Covishield, use a chimpanzee adenovirus, called ChAdOx1. Another adenovirus-based vaccine, produced in China, relies on human adenovirus 5 (Ad5). Ad5 is the most common type to infect people, so researchers need to balance its ability to generate an immune response with high levels of pre-existing antibodies in some populations. These concerns are often the reason that scientists opt to use other types of adenovirus that less frequently or never infect people.

The Johnson & Johnson vaccine was submitted to the U.S. Food and Drug Administration (FDA) for consideration as a single dose vaccine; however, ongoing studies will evaluate the utility of adding a second dose 56 days after the first. Of those currently approved for use in other countries, the AstraZeneca and Covishield vaccines are given as two doses at 0 and 28 days. Sputnik V is given as two doses at 0 and 21 days, and CanSino, the China-produced vaccine, is given as a single dose. All are given intramuscularly.

Making adenovirus vector vaccines

Most often to be used as a vaccine vector, adenoviruses are rendered replication-deficient, meaning they cannot reproduce in vaccine recipients. This is accomplished by removing two genes that the adenovirus needs to reproduce and replacing it with the gene for the protein of interest — in this case the SARS-CoV-2 spike protein.

Because viruses need cells to reproduce and the altered virus cannot reproduce, scientists need a way to make large volumes of the altered vaccine virus. This is most often accomplished using cell lines that have been modified to include the gene that was removed from the adenovirus. While a handful of options are available, the two most common are:

• HEK-293 — This human embryonic kidney cell line, originally isolated in 1972, was altered to include the adenovirus reproduction gene in the late 1970s. While a popular choice, on occasion, this cell line allows for production of small quantities of adenovirus that have regained the ability to reproduce. Because researchers are aware of this tendency, they can check for it, and in 2001, the FDA developed guidelines to standardize the amount of live virus that can be present in a sample. However, to avoid this concern, some companies opt to use a different cell line.
• PER.C6 — The most popular alternative, this cell line, originally isolated in 1985, was developed from human embryonic retinal cells in the late 1990s. While these cells are less likely to restore the ability to replicate, this can still occur on occasion, but is much less of a problem. However, this cell line presents its own issues to overcome, including stricter and more costly licensing and difficulties adapting to growth in serum-free cultures.

Regardless of which cell line is used, once large quantities of the virus are grown, the vaccine virus is purified through a series of steps before being prepared for distribution. In terms of approved (or submitted) COVID-19 vaccines, Johnson & Johnson used the PER.C6 cell line; all others used HEK-293.

Because the adenovirus-based vaccines use fetal cells during production, some individuals may not be willing to receive these types of vaccines. The good news with the COVID-19 vaccine situation is that, unlike some other vaccines that rely on fetal cells during production, the availability of alternative vaccines will enable people to be protected against COVID-19 even if they are not comfortable receiving a vaccine made using fetal cells.

Mechanism of action

Adenoviruses enter muscle cells and cells of the immune system, so viral proteins are produced by both cell types, causing direct processing of spike protein by immune system cells as well as activation of immune system cells by infected muscle cells. Some parts of the immune response may also develop at more distant sites, such as the spleen.

Unlike the mRNA vaccines, adenovirus vaccines deliver instructions by DNA. This difference means that when the DNA-based adenovirus vector is delivered to a cell, the DNA makes its way into the nucleus, where it serves as a blueprint to produce mRNA that is released into the cytoplasm. The mRNA is then used to guide production of coronavirus spike protein, which is ultimately processed by immune system cells to generate immunity.

The delivery of DNA dictates two important considerations:

1. Genetic implications — Hearing that the DNA enters the nucleus will naturally lead some to wonder whether the vaccine could cause genetic changes. The good news is that it cannot. Adenoviruses are not incorporated into our DNA during infection because they do not contain integrase, an enzyme necessary for incorporation. So, even though the DNA is in the nucleus, it does not mix with, or alter, our DNA. Indeed, we are infected with DNA viruses like adenovirus all the time. These encounters never result in a change in our DNA.
2. Longevity — While mRNA is broken down relatively quickly in our cells, DNA is not as quickly removed. This situation explains both the increased immune response over subsequent weeks after vaccination, and the reason that some of these vaccines can be given as a single dose. Because the DNA continues to make mRNA, the cells produce the spike protein longer, which means that the immune system is exposed to antigen for longer periods of time. As a result, the immune response has time to mature and strengthen during this prolonged period. In phase I studies by Johnson & Johnson, immune responses were detected by most participants in the 65 and older cohort two weeks after vaccination, but even more had detectable responses about one month after vaccination. Similarly, adults 18 to 55 years of age had higher average antibody titers, including neutralizing antibodies, around two months after vaccination compared with those measured one month after vaccination.

Immune responses

Adenovirus vector vaccines stimulate both innate and adaptive immune responses and are inclusive of both T cell and B cell responses, including production of memory. Further, the type of T cell responses that develop favors those that typically follow viral infections, known as Th1 responses. As a result, vaccine-induced immunity is expected to be robust; however, trial participants will continue to be monitored, as with those in the mRNA vaccine trials.

It is also important to realize that in addition to generating immunity to the SARS-CoV-2 spike protein, it is likely that immunity is also developing against the adenovirus vector proteins. What this means over time if additional vaccines use the same vector remains to be seen. Given that people with pre-existing immunity to Ad5 have had lower immune responses to vector vaccines with this backbone, it is possible that future vaccines could have a similar finding. Alternatively, the concern could be specific to Ad5, which was suggested by data evaluating increased HIV susceptibility in men following receipt of an Ad5-based HIV vaccine. For this reason, some have suggested that Ad5 vectors should not be used for COVID-19 vaccine development.