Why Covid-19 Vaccines Work Well Against Delta Variant

Vaccines designed to target earliest virus version are holding up well against new contagious strain, largely due to the shot’s design.

Research is ongoing but suggests that authorised Covid-19 vaccines, though somewhat diminished in efficacy against Delta, can still protect people from severe cases. Picture: AFP

Covid-19 vaccines, designed to target the earliest version of the coronavirus, are holding up well against the Delta variant because it isn’t so different from the original strain that it can evade the shots, researchers said.

Yet the Delta variant is different enough, the researchers said, that studies indicate people will need both doses of the messenger RNA shots or a booster for the one-dose vaccines to get the kind of immune response necessary to provide full protection.

The Delta threat

The Delta variant is the most contagious version of the virus to be identified. In a matter of weeks, it has become the most common strain in the US, now accounting for 83% of analysed infections.

Research is ongoing but suggests that authorised Covid-19 vaccines, though somewhat diminished in efficacy against Delta, can still protect people from severe cases.

Saying vaccination is the best defence against Delta, health authorities have been urging people to get immunised.

“You might test positive, but the vast majority of people who are fully vaccinated are not going to get very sick from it or sick from it at all, ” said Dr. Angela Rasmussen, a virologist at the University of Saskatchewan.

There you have it in @NEJM.

Pfizer is ~90% effective against delta. AstraZeneca is ~70%. This is similar to effectiveness against other variants, including alpha.

Vaccines work, even against variants.

Get vaccinated if you haven't already.https://t.co/FV1Rr5fiVa

— Dr. Angela Rasmussen (@angie_rasmussen) July 22, 2021

How vaccine design works to combat Delta

The shots’ powers, against a version of the virus that wasn’t around when researchers were developing Covid-19 vaccines last year, reflect their design targeting large segments of the coronavirus spike protein.

The spike protein, which juts from the surface of the coronavirus, is a distinguishing feature of the pathogen and the object of vaccine developers seeking to mobilise immune-system defences.

Like most vaccines, the Covid-19 shots work by tricking the body into thinking it has been infected with a virus, training the immune system to make molecular weapons to fight when a real pathogen comes.

The vaccines generate neutralising antibodies, which prevent the virus from entering cells and replicating. The antibodies seek to stop the virus by binding to the spike protein, thereby blocking it from infecting cells.

Segments of the spike protein change as the virus mutates into new variants. That makes it harder for antibodies generated by the vaccines to bind to the spike protein because the antibodies might not recognise some mutated parts.

“It’s like you are training a pilot on one kind of a plane and a slightly different model shows up for the actual flight,” said Dr. Saad Omer, director of the Yale Institute for Global Health.

But because the vaccines target a large portion of the protein, so far none of the small variants, including Delta, have rejiggered the spike protein enough to elude vaccine-generated immune defences.

Why two doses are needed

The number of antibodies generated in fully vaccinated people is still so high that enough antibodies can bind to the spike protein and block the virus, researchers said.

Two doses are needed, researchers said, because neutralising antibodies don’t work as well after just one dose, and the body needs a higher level of antibodies given the presence of mutations.

Also, the greater levels of neutralising antibodies triggered by the two doses create a stronger immune response, virologists said.

“The second dose is going above that threshold of protection, whereas the first dose alone is inducing a suboptimal level of antibody that is just not enough to protect you from infection,” said Dr. Akiko Iwasaki, an immunologist at Yale University.

A second dose also boosts the numbers of two other immune-system weapons: the T-cells that hunt down infected cells and destroy them, and memory B-cells, which circulate in the blood and help churn out antibodies upon detecting a virus.

Both are likely helping prevent severe disease, said Shane Crotty, a virologist at La Jolla Institute for Immunology in California. For example, T-cells recognise viral cells in more ways than neutralising antibodies and likely retain strength against variants, he said.

“Those T-cells will grow like crazy, and the B cells will grow like crazy to stop the virus,” Dr. Crotty said.

Once a body recognises it has been infected, the cells go to work, clearing out the virus before severe disease can develop.

“It basically happens immediately with a vaccinated person, whereas it can take up to two weeks to develop in an unvaccinated person,” Dr. Rasmussen said.

Yes. You can get COVID after vaccination but it’s typically mild disease

With Without
Vaccination Vaccination pic.twitter.com/GIcb5mJQz9

— Faheem Younus, MD (@FaheemYounus) July 18, 2021

Vaccines from Pfizer Inc. and partner BioNTech SE and from AstraZeneca PLC were highly effective against the Delta variant after two doses, according to a study of nearly 20,000 people published Wednesday in the New England Journal of Medicine.

After two doses, the Pfizer vaccine was 88% effective against Delta, compared with 36% after one dose, said the study, which was sponsored by Public Health England. Meanwhile, the effectiveness of AstraZeneca’s shot was 67%, compared with 30% after one dose.

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