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)

But statistics about breakthrough infections are promising. Breakthrough infections are infections that occur in vaccinated people, so it’s when people get infected with SARS-CoV-2 despite having received a Covid jab. 

Data from England shows one Pfizer jab offers 36% protection against mild infection from the Delta variant, and two shots reduce your chances of infection by 88%.

The Delta variant is the form of the virus which is currently dominant in South Africa.

Real-life data from the US government’s Centres for Disease Control reveals that fully vaccinated people are five times less likely to get infected with the Delta variant than those who are unvaccinated. 

Last, the WHO says governments must consider the ethics of supplying booster jabs to their populations when much of the developed world is still waiting for jabs to complete the first round of their roll-outs.

“Offering booster doses to a large proportion of a population when many have not yet received even a first dose undermines the principle of national and global equity,” the WHO warns.

South Africa has bought enough doses to immunise 40-million people and has fully vaccinated 19.5% of its population so far. 

Covid in countries that are rolling out boosters 

In July, Israel became the first country to introduce Covid boosters for those over 60. Less than a month later, the extra shots were available to anyone over 30. When the country started to roll out boosters, 62% of the population had been fully vaccinated, yet there was a surge of Covid cases. 

Scientists were concerned that the rise in infections was because of the highly infectious Delta variant circulating in the country and the fact that the protection offered by vaccines was possibly waning. 

An August preprint study from Israel showed that immunity had begun dropping in vaccinated people six months after they had been fully immunised.

More Israel data was published in the New England Journal of Medicine (NEJM) in October, showing that six months after a second Pfizer vaccine jab, the immune system’s antibody response substantially decreased, especially among trial participants who were men, people older than 65 and those with weakened immune systems.

They also found that antibody levels were stronger in people who had previously recovered from Covid and then been given two Pfizer shots, compared to those who had never been naturally infected and only been vaccinated. This, researchers argue, could possibly indicate the benefit of people receiving a third booster shot. 

What do we know about boosters?

In October, data from this booster roll-out was published in the NEJM. Israel’s approach to giving third doses of Pfizer’s vaccine helped lower the chances of people getting infected with SARS-CoV-2 and developing severe illness.

The NEJM paper shows that 12 days after receiving the third shot, people over 60 were almost 20 times less likely to develop severe Covid than those who had received the standard two-dose vaccine. Study participants who were given boosters had been fully vaccinated at least five months earlier.

Following Israel’s lead, several countries began rolling out booster programmes.

In September, the US Food and Drug Administration approved a third Pfizer shot for everyone aged 65 and older, as well as for those who may be younger, but at high risk for severe disease.

In October, the FDA also cleared the Moderna and J&J vaccines to be used as booster shots. (The Moderna vaccine is not part of South Africa’s vaccine roll-out.) 

Moreover, the US regulator approved the “mix and match” use of vaccines. This means people who received Pfizer’s two-dose vaccine initially would be able to get a J&J or half a dose of Moderna vaccine for their third dose.  

Those who got J&J’s one-dose vaccine can likewise be given one Pfizer shot as their third dose. 

Third doses should be administered at least two months after a person has been fully vaccinated with one dose of J&J, or six months after two doses of Pfizer, the statement reads. 

This decision followed data from a preprint study which showed that people who started with a J&J shot were better off with a Pfizer or Moderna jab as their booster. 

For people who had Pfizer or Moderna in the first round of vaccinations, it didn’t make much of a difference whether they got a J&J, Pfizer or Moderna booster — they all worked equally well. 

South Africa’s Health Minister, Joe Paahla, said in October that the health department will offer Pfizer jabs as second doses to people who had received a first jab of the Moderna or AstraZeneca vaccines in a different country. Sahpra has, however, not yet approved a mix and match policy for South Africa. DM/MC

This story was produced by the Bhekisisa Centre for Health Journalism. Sign up for the newsletter.