Uttam Maharjan

Booster programmes have been initiated, or are underway, across the world. In Nepal too, the programme has been going on for the last few weeks. Frontliners, those above sixty and those with a compromised immune system, are receiving booster shots now. It was believed until a few months ago that a person should receive vaccine doses – first, second and booster doses – of the same type. However, for reasons like supply constraints, a mix-and-match approach has been adopted. As per WHO and other guidelines, the government has announced the Covishield vaccine as the common booster jab for those who have received any vaccine, whether Covishield, Vero Cell, Johnson and Johnson or Pfizer.

Several studies and researches have shown that the mix-and-match approach to vaccination is effective at boosting the immunity of people having received primary jabs of a different type. It has also been revealed that administering different jabs for the first and second doses also induces a strong immune response against COVID-19. The side effects resulting from the mix-and-match regimen are not worse than those generated by the standard regimen. If anything, mixing the first and second doses is more effective than the regime of the same doses. Researches have shown that the mix-and-match regimen could be as good as, or better than, that of mRNA vaccines.

Immune responses
The high antibody levels, coupled with strong immune responses, ensure that the mix-and-match regime is something to reckon with. However, researches into the mix-and-match approach to booster shots are in the early stages. Even in the US, a major producer of vaccines, heterologous booster shots have been administered to billions of people. As per the UK Cov-Boost study published in Lancet in December 2021, six different vaccines are safe and effective at producing strong immune responses when administered as booster shots after the administration of two doses of either AstraZeneca or Pfizer-BioNTech vaccines. In the study, seven vaccines were studied: Oxford-AstraZeneca, Pfizer-BioNTech, Novavax, Johnson and Johnson, Moderna, Valneva and Curevac.

As per the randomised Phase II trials, all the aforementioned vaccines are safe to use as third doses. However, immune responses vary widely. All the vaccines boost spike protein immunogenicity when administered after two doses of AstraZeneca, while all the vaccines except Valnera are effective after the administering of two doses of Pfizer-BioNTech. As per the analysis carried on by Peter Nordstrom, a Swedish epidemiologist, and others, compared with unvaccinated people, those receiving a mix-and-match regimen are 68 per cent less likely to catch COVID-19 vis-à-vis those receiving a homologous regimen of two doses who are 50 per cent less likely to contract the disease.

A similar analysis in Denmark carried on by epidemiologist Mie Agermose Gram and others shows that a heterologous regimen of AstraZeneca followed by Pfizer-BioNTech is 88 per cent effective at keeping COVID-19 at bay. Studies find that different vaccines are effective as booster shots in their own ways. This means one vaccine may be more effective than the other. However, one thing is dead sure: booster shots work effectively at boosting immune responses against the coronavirus. The US Food and Drug Administration approved heterologous booster shots in October 2021. As such, the booster campaign has been going on in the US without a letup.

The mix-and-match regimen has proved to be effective at preventing COVID-19. But there may questions such as for how long the effectiveness of the regimen may last or when boosting with extra doses is required. The Nordstrom team has found that the durability of the mix-and-match regimen is similar to that of mRNA vaccines. The mix-and-match regimen will have greater benefits for the medical community. On the one hand, adopting this approach can steer clear of supply-chain bottlenecks, while on the other it can give better protection to people. There is evidence that the mix-and-match regimen can elicit a stronger immune response in immunocompromised people than a homologous regimen can. Organ transplant patients and others with a weakened immune system can greatly benefit from the mix-and-match regimen.

The inoculation campaign has been going on all over the world. The major concern is how to keep the coronavirus at bay. The WHO, in association with its partners, is working on the COVAX facility for the fair and equitable distribution of vaccines in both developed and developing countries. Still, the goal of evening out the vaccine distribution has not been successful. The poorest countries have not been able to lay their hands on vaccines to inoculate their eligible populations.

Vaccination drive
It is surprising that despite the vaccination drive picking up momentum, COVID-19 has not shown any signs of abatement. Rather, some Pacific island countries that have relatively less infection rates over the last two years have come into the grip of the disease in recent times. This has shown that the disease cannot be controlled until people have developed herd immunity. Herd immunity can be developed in people through vaccination as well as through the observance of health safety protocols.

The mix-and-match regimen has proved to be a boon in the fight against COVID-19. But for the regimen, it would have been hard to arrange for homologous regimens. In Nepal too, it is hard to manage homologous booster shots. Those who have received Vero Cell as first and second doses are also receiving Covishield vaccines owing to the limited stock of Vero Cell vaccines. Elderly and immunocompromised people may benefit from the mix-and-match regimen as it is more effective for them than the homologous regimen.

(Maharjan has been regularly writing on contemporary issues for this daily since 2000. uttam.maharjan1964@gmail.com)