What are “viral vector” vaccines?

Two more COVID-19 vaccines have entered into Phase III human trials this month in the US. These vaccines use “viral vectors.” Viral vector vaccines work differently than the Moderna and BioNTech mRNA vaccines.

What are viral vector-based vaccines?

Vector-based vaccines stimulate the body’s immune system by using a weakened form of a virus to deliver the part of a pathogen the immune system recognizes.  In other words, the vector (weakened virus) is a distribution mechanism for an antigen (small piece) of the pathogen (germ) the vaccine is designed to protect against. Your body sees the antigen and develops an immune response to it, protecting you from infection when you come in contact with the pathogen itself. Commonly, adenovirus (Ad) vectors have been used. These are inactivated common cold viruses. The new candidate viral vector vaccines have been shown to stimulate robust immune responses. Currently, there are no approved adenoviral vector vaccines in the US.

The Johnson & Johnson Candidate Vaccine

One candidate vaccine uses a vector called Ad26. It’s produced by Johnson & Johnson and enters Phase III this month. Like all the other candidate vaccines,  Ad26 delivers the SARS-CoV-2 spike protein gene into cells. The cells then make the spike protein themselves, which stimulates an immune response. Findings published in Nature showed the vaccine protected macaques with just 1 dose. One challenge is that many people have pre-existing immunity to Ad26, meaning that the immune system may stop the vector before it can gain entry into the target cells.

The AstraZeneca Candidate Vaccine

To get around possible pre-existing immunity, a second vector vaccine, produced by AstraZeneca, uses a chimpanzee vector Ad5 to deliver the spike protein gene. It went into Phase III in the US on August 31, 2020. Initial results in Nature showed promise in animals. The human trial uses two doses of the vaccine. 

On September 8, the trial was placed on immediate hold  following a serious illness in a participant. It is standard practice for researchers to place trials on hold if there is unexpected illness in a participant, called a “serious adverse event”.  While the trial is on hold, doctors will investigate to determine if the adverse event is related to the vaccine or not.  Until we know whether or not the vaccine was connected to the participant’s illness, there is no way to know if this will impact the safety of this vaccine. It is actually reassuring when stoppages like this happen; they are evidence that scientific safeguards to find a safe vaccine are working. 

Last update: September 9, 2020 08:00 am ET

Science review: GSN, JAB