South Africa looks at rolling out boosters for the already vaccinated

Nine months after nearly half a million healthcare workers in South Africa were jabbed with a single dose of the Johnson & Johnson vaccine (J&J) in the Sisonke implementation trial, they are likely to get a second shot of the same vaccine in a new leg of the study.

The Sisonke study ran from February to May and immunised 496,900 healthcare workers with  J&J’s vaccine. The programme used leftover stock from J&J’s international vaccine trials and ran when South Africa had no other jabs available. The local study’s goal was to provide protection to South Africa’s frontline workers.

A second, booster dose is, however, not a sure thing yet — the country’s medicines regulator, the South African Health Products Regulatory Authority (Sahpra), is currently reviewing data submitted by Sisonke’s researchers to decide whether it is safe and potentially effective for the trial’s participants to get another shot up to eight months after their first jab.

What is a booster shot?

Each Covid vaccine comes with its own recommended dose. For instance, the J&J jab requires only one dose, which means that, at this stage, you are considered fully vaccinated after a single shot. However, those who get the Pfizer vaccine are only considered completely immunised after getting two jabs.

Now, countries like the United States and Israel have decided to give an extra shot to some of those who are already fully vaccinated. These are called “booster” shots.

For now, boosters are mostly given to specific groups of people. Extra shots are especially important for people who have weakened immune systems, such as those who have undergone a bone marrow transplant or who are older than 60.  

People who have had organ transplants use medication that suppresses their immune systems to prevent their bodies from rejecting the organs, and older people’s immune systems respond less effectively to foreign invaders in their bodies than those of younger people. 

Such individuals may therefore not be able to build up enough protection after receiving the standard number of vaccine doses.

How long do Covid vaccines protect you? 

Waning immunity is not a new concept — but it also may not be a reason to panic.

Childhood vaccines, such as those used for protection against measles and mumps, have shown a yearly drop of between three and 10% in the amount of antibodies that people produce against those diseases after vaccination. 

In the case of Covid, vaccines protect people against getting very sick with the disease, but we don’t yet know how long that protection lasts, and the strength of the protection remains unclear. 

For the Pfizer vaccine, there’s some data that shows people are protected against symptomatic Covid for about six months after having received two shots, but scientists don’t yet know for certain whether the fact that people produce fewer antibodies against SARS-CoV-2 (the virus which causes Covid-19) six months after they have been fully immunised with certain jabs, translates into weaker protection against the virus.

Here’s why researchers don’t yet have an answer for this.

Your immune system consists of complex defence systems and some of these responses become weaker as time moves on after vaccination. Each part of your system is responsible for a different type of attack on foreign invaders such as viruses or bacteria. 

Your immune system produces two types of cells to get rid of germs, attack them and kill off infected cells. They are called B cells and T cells. 

B cells make antibodies that fight against the invading germ. T cells, on the other hand, fight and remove cells that have already been infected by the invading germ. When you’re exposed to SARS-CoV-2, your body has to fight off the virus using these internal defence systems.

Your immune system’s ability to “remember” germs it’s come across in the past is an important part of why vaccines are so successful.

Back in May, a group of Australian researchers published a paper in Nature Medicine predicting that after eight months, the protection provided by Covid vaccines against infection would drop — but they would still be protected from severe disease.

That’s because of memory cells, which can be T or B cells, but we’re only looking at B memory cells in this article. 

Memory B cells make antibodies when they come across a familiar foe. Although the number of antibodies people produce may decrease from about six months after vaccination, memory cells usually stick around for longer. 

An October paper in Science found that memory B cells triggered by mRNA vaccines increased six months after people had been fully immunised. This protection also held strong for the Alpha, Beta and Delta variants of the virus. The mRNA Covid jabs on the market are Pfizer and Moderna’s shots (South Africa uses Pfizer).

The increase of memory cells over time in those who have been vaccinated seems to mirror similar findings in people who were naturally infected with SARS-CoV-2 and recovered from Covid. In a February study in Science, their antibodies began dropping eight months after infection, but memory B cells increased.

So even though people may be losing some antibodies a few months after vaccination, it doesn’t necessarily mean that the jabs aren’t doing their job.

In summary: your immune system has many different prongs through which to protect you, and antibodies are only one part of that. The long-lasting presence of memory B cells and the continued protection vaccines grant against severe disease means that boosters may not be entirely warranted just yet.

How does a country decide if it needs boosters?

The need for boosters depends on what a country’s Covid outbreak looks like and which vaccines are in use.

The World Health Organisation (WHO) says there are three things nations should look at when considering booster shots. The first is waning immunity. The second is how long vaccines protect against specifically mild forms of Covid infection — in other words, the likelihood of getting infected with SARS-CoV-2. Most of the research that looks at protection against mild Covid has, so far, been conducted in unvaccinated people.

What scientists now need to find out is how well vaccinated people are protected against mild Covid disease and if that protection changes over time.  

A nurse injects a man with a third shot of Covid-19 vaccine in Jerusalem, Israel. (Photo: EPA-EFE / Abir Sultan)