Mixing different types of COVID-19 vaccines could help boost the immune response, but the idea has been slow to take hold. Angela Weiss / AFP via Getty Images Hide caption
Angela Weiss / AFP via Getty Images
Angela Weiss / AFP via Getty Images
If you are given a COVID-19 vaccine that requires two doses, you should usually be given two of the same vaccine. Two Pfizer shots or two Moderna shots. Not one then the other.
But in the future this could change either by necessity or by design.
This idea of using two types of vaccines is not a new concept. It is known as heterologous vaccination, although there is a more slang term.
“In the UK we currently call it mix and match,” he says Helen Fletcher, Professor of Immunology at the London School of Hygiene and Tropical Medicine. She says a shortage of vaccines or concerns about side effects could lead health officials to adopt a mix-and-match strategy.
Health authorities in France and Germany are already encouraging people who have received the AstraZeneca vaccine to consider receiving one of the vaccines mRNA vaccines for her second shot.
“So there is a practical reason you might want to mix two different types of vaccines. But there is also a scientific reason,” says Fletcher.
Basically, all vaccines work by showing people’s immune systems what looks like an invading virus but really isn’t. If the real virus ever occurs, its immune system will recognize it and prepare it to fight it off.
Using two different vaccines is a bit like giving the immune system two pictures of the virus, maybe one in-person and one in profile.
“If you give two different types of vaccines, you tend to get a better immune response than if you give the same vaccine twice,” says Fletcher.
Some vaccine manufacturers have adopted this approach and make vaccines of two different types.
One is a company called Gritstone Bio based in Emeryville, California.
“The natural human response to a virus is to mobilize two different arms of the immune system,” says Gritstone CEO Andrew Allen. One uses antibodies, the other relies on something called a CD8 T cells. Unlike antibodies, CD8 T cells do not directly recognize a virus, but they do recognize a cell that has been infected with a virus and they can destroy the infected cell.
Gritstone has developed two different vaccines to activate each arm: a vector viral vaccine and an mRNA vaccine. The vector viral vaccine stimulates CD8 T cell production very well.
“The mRNA [vaccine] makes a really good antibody response. When you combine these, you might get the best of both worlds, “says Allen.
Gritstone’s approach is already being tested on human volunteers.
This mix-and-match approach has been tried with vaccines against a wide variety of diseases.
“Diseases like HIV, malaria, TB, even influenza,” says Bali Pulendran, Violette L Horton Professor and Professor of Immunology and Microbiology at Stanford University. “So there is ample evidence that such strategies are beneficial.”
If it’s such a good idea, why isn’t it used routinely?
Pulendran speculates that there are two main reasons. You have to do with how new vaccines are approved.
“Regulators love the simplicity,” says Pulendran. “The simpler the vaccination schedule, the tastier they’ll find it.”
The other reason is that while mix and match makes scientific sense, it doesn’t always make business sense.
“For example, if Company A makes one vaccine and Company B makes another vaccine, unless there is an overarching incentive for the two companies to have some kind of marriage, either company would generally prefer to join.” “Says Pulendran.
Of course, when a small company like Gritstone shows that a mix-and-match strategy really does lead to a dramatically better vaccine, you can bet that other drug companies will find a way to solve the business problem.