Are Covid-19 vaccine boosters really effective?

Senior citizens and those with chronic diseases are highly encouraged to get a second Covid-19 booster shot to help maintain their protection against severe disease and death. — CHAN BOON KAI/The Star

With the recent announcement that the wearing of face masks is no longer compulsory in indoor settings, things have taken a further turn towards the “old normal” after around two-and-a-half years of precautionary measures to avoid the spread of Covid-19.

The development and rollout of vaccines against the SAR-CoV-2 virus played a huge part in allowing this to happen.

According to the CovidNow website, the large majority of Malaysians have completed their two primary vaccine shots – 86% for the first dose and 84.2% for the second dose as of Sept 23 (2022).

This number falls by a significant amount however, for the first booster dose, which is recommended for all eligible age groups.

Only about half (49.7%) of Malaysians has received their first booster dose as of Sept 23 (2022).

It falls even more dramatically for the second booster dose, which is currently recommended for senior citizens and high-risk individuals, e.g. those with chronic diseases and/or are immunocompromised.

Only 1.5% of the population has received a second booster as of Sept 23 (2022).

One question many may have is the effectiveness of the booster shots, especially against newer viral variants like Omicron, which didn’t even exist when the vaccines were first developed.

University of the Philippines Infectious and Tropical Disease Division chief Professor Dr Anna Ong-Lim notes that there is a difference between efficacy and effectiveness.

Speaking online to regional media, she says: “We hear these words so often now and I think we have the impression that they mean the same thing, but they are really different concepts.

“In research, we talk about efficacy – it’s an outcome that is measured in a clinical trial setting.

“Generally, this means that there are very tight controls applied on what processes happen and how the participants are selected.

“And because the parameters are really very clearly defined, we can be quite certain that whatever outcomes are measured, are due to the conditions that are applied in that setting.

“So, efficacy data is critical as a basis for securing approval from regulators, because of course, regulators are focused on making sure that these formulations are safe and they work as they are claimed to be working.”

On the other hand, she shares that effectiveness is the reflection of how a vaccine actually works in the real world, where things are not as tightly controlled as in a clinical trial.

For example, vaccine recipients would have much more variable genetic and physiological characteristics, while dosing schedules may not be strictly followed for various reasons.

The Philippines Health Department Technical Advisory Group member says: “We’ve heard people talk about vaccine effectiveness of 80% or 90% or 70%.

“And we seem to think that this is reflective of how well that vaccine is expected to work, when in fact, this is referring to the percentage reduction of the expected outcome.”

She explains that this means for a Covid-19 vaccine that is 80% effective in protecting against severe disease and hospitalisation, out of 10 people who receive the vaccine, eight will be prevented from developing severe Covid-19 that requires them to be hospitalised.

Mixing it up

Prof Ong-Lim was one of 22 subject experts recruited from across Asia and Latin America to review the latest data gathered from real-world settings on the effectiveness of Covid-19 vaccines during the current SARS-CoV-2 Omicron viral variant wave.

Local consultant infectious diseases physician and retired Health deputy director-general (research and technical support) Datuk Dr Christopher Lee was also one of the expert reviewers.

The findings were published online early this month (September 2022) as a preprint – meaning that it has not yet been peer-reviewed – at the Research Square website.

Overall, the vaccine effectiveness (with one booster) against severe Covid-19 and death was 84.2% for the general population and 87.4% for those aged 60 and above.

Says Prof Ong-Lim: “We saw that regardless of the platform used for boosting (i.e. whether via mRNA, an inactivated virus or a viral vector), the effectiveness for these two groups was quite high if the outcome we were measuring was protection against severe disease and death.

“So this is quite reassuring, especially because one of the limitations in lower- and middle-income countries is really that we have to work with what vaccines are available in our systems, and there might be a perception that some vaccines are better than others.”

She also shares that both homologous (where the same vaccine is given for all shots) and heterologous (where the primary and booster vaccines are different) vaccine schedules showed the same level of effectiveness and protection against severe Covid-19 and death (see interactive graphic below).

This, she says, is very relevant for the South-East Asian region where most people have received different vaccines for their Covid-19 shots.

The study did find however, that the protection against infection and mild disease decreases quite quickly after a booster shot.

Two out of three people (67.1%) will be protecting against getting infected and having mild Covid-19 within two weeks of getting their booster shot.

By about three months, this will go down to one in two people (49%), and after three months, it further decreases to under one in three people (31.9%).

But the good news is that the protection against severe disease and death only decreases slightly from 87.5% within the first two weeks after a booster shot, to 77.5% after three months.

This, according to Prof Ong-Lim, is really reassuring as this is the outcome that the vaccines were designed to produce in the first place.

Maintaining protection

Meanwhile, a study in Chiang Mai, Thailand, found that protection against severe outcomes from Covid-19 – defined as requiring ventilation or death – was 93% between two weeks and three months after receiving the first booster.

It went down slightly to 87% between three to six months after the shot.

This study involved around 300,000 confirmed Covid-19 patients infected with either the Delta or Omicron viral variants, who had been vaccinated.

Published as a preprint on the Research Square website on Aug 26 (2022), the study found that three doses of Covid-19 vaccines – mostly with mixed vaccines – provided high levels of protection against severe Covid-19 and death (see interactive graphic below).

“This is very important for the people, and also the public healthcare facilities, because you can prevent people from developing serious disease.

“Most people, if their symptoms are mild, can be taken care of at home and hospital beds can be kept for moderate or severe patients who really need the ICU or the ventilator,” says study co-author and Chiang Mai University Faculty of Public Health dean Emeritus Prof Dr Suwat Chariyalertsak.

He notes that no one who had received a second booster shot developed severe Covid-19 or died from the infection during the period of the study from February to April (2022).

In total, there were 175 severe cases of Covid-19 in Chiang Mai during that period, most of which were among people who had not been vaccinated at all, shares the infectious disease epidemiologist.

He says that the data shows the importance of getting all those eligible – but especially those above the age of 60 and those with chronic illnesses – to receive at least three doses of the Covid-19 vaccine, in whichever combination of vaccine types, for the best level of protection against severe disease, hospitalisation and death.

Prof Chariyalertsak adds that this too is important to help public hospitals maintain enough space and resources to treat patients with other serious illnesses.

Get your booster

Dr Lee, who is a member of the Selangor Public Health Advisory Council, opines that the most striking thing about the Covid-19 vaccines is that they work, and they work very well.

“Yes, we heard that as time goes on, the vaccines may not be as effective in preventing an infection, but what is very heartening is to look at the data with regards to severe disease and deaths – that has been really remarkable,” he says.

He notes that boosters, whether they be of the same type as the first two shots or not, appear to work well when they are a viral vector or mRNA vaccine, even with the Omicron viral variant.

However, the decrease in their effectiveness over time is a worry.

“As the experience in Thailand has shown, by six months or so, we are concerned that the immunity may not be strong enough, and the fourth shot or second booster is something that is on the cards,” he says, adding that many countries are now offering this second booster to those aged above 60 and those with chronic illnesses.

As to how long the effectiveness of a second booster will last, it is something that only time can tell as it has not been long since these fourth shots have been offered, he says.

Dr Lee also notes that the Omicron viral variant is unlikely to be the last variant of SARS-CoV-2 we will see.

“What is certainly very encouraging is that the vaccines seem to still work very well in preventing severe disease, even with Omicron,” he says.

He adds that this is an important consideration for those who might be awaiting the arrival of the Omicron-specific vaccines that were approved for emergency use by the US Food and Drug Administration (FDA) on Aug 31 (2022).

“For most of us in this part of the world, our turn to get these variant-specific vaccines may take quite a bit of time.

“So I think as we deep dive into all these studies that are linked to Omicron, it is important not to delay our vaccination schedules.

“Let us not wait for these variant-specific vaccines, which may take a long time to come, but use what we have first,” he says.