Explained: Why COVID-19 booster shots and vaccine-mixing have only added to WHO’s worries

The World Health Organisation (WHO) chief, Tedros Adhanom Ghebreyesus said that the four millionth recorded death worldwide from COVID-19 was a ‘tragic milestone’ which ‘likely underestimates the overall toll’ of the deadly virus. AP

Tedros Adhanom Ghebreyesus, the World Health Organisation chief, has spoken out against countries procuring booster doses of COVID-19 vaccines for their citizens while their less fortunate counterparts had yet to vaccinate essential workers and vulnerable populations.

But as countries let their hands be guided by fear or desperation, news also emerged of Thailand deciding to mix two types of vaccines as it finds itself caught amid an alarming surge in cases. The UN health agency has urged caution on this, too, although multiple tests have tried to explore the benefits of ‘mixing and matching’ Covid vaccines.

Why are countries ordering booster shots?

US pharma giant Pfizer has said it will seek emergency authorisation for a third, follow-up dose to its two-dose-regimen mRNA COVID-19 shot as it has found that the immunity conferred by its vaccine wanes over time even as new variants may be better suited to dodge the antibodies created by the jab.

“The Pfizer vaccine is highly active against the Delta variant,” the company’s chief scientific officer, Mikael Dolsten has been quoted as saying. But he has warned that six months down the line, “there likely is the risk of reinfection as antibodies, as predicted, wane”.

A reference point would be the example of Israel. The country reported a dip in the Pfizer vaccine’s effectiveness and Dolsten put that down to infections in people who had been vaccinated in January or February.

News agency Reuters said that the Israeli health ministry reported that vaccine effectiveness in preventing both infection and symptomatic disease dropped to 64 percent in June. However, at the same time, the vaccine was 93 per centeffective against preventing hospitalisations and serious illness.

Even so, Israel appears to be not taking chances and has said it will start giving a third vaccine shot to people with weak immune systems even though it is still not decided whether it will throw open boosters for the entire population.

Health officials in the US and European Union, meanwhile, seem to be sure that a third shot is not needed at the moment. The US drugs and diseases watchdogs have said in a joint statement that people who have received both doses of their COVID-19 vaccines do not need a booster shot at present. The country has been using the Pfizer-BioNTech and Moderna mRNA shots and the single dose Johnson & Johnson viral vector vaccine.

The European Medicines Agency (EMA), too, has said it’s still early days to take a call on whether more than the two shots are currently required.

What’s the vaccination situation around the world?

Amid this debate on boosters shots, poorer countries have got left behind in the vaccine procurement race. A Duke University tracker says that “many countries in Latin America, Africa, and Asia still have not yet been able to purchase enough vaccine to cover their populations” while noting that “by the end of the summer 2020, the UK, EU countries, and Canada had purchased enough doses through advance market commitments to cover more than their entire populations”.

Experts explain that manufacturing pressures mean that prior deals made by the richer countries leave only “a smaller piece of the pie” for low- and middle-income countries and for the likes of the COVAX partnership, which is working to ensure equitable vaccine distribution.

According to the tracker, high and upper middle income countries have so far procured or booked close to 8.5 billion doses of Covid vaccines while lower middle income and low income countries have managed to get hold of a little over 2 billion doses. Which means that the vaccination process globally is moving along very different tracks for the rich and the not-so-rich countries.

But as travel opens up and the countries thirst for economic action, the existence of the virus anywhere will continue to be a threat for societies that have achieved full vaccination. Which is what has prompted experts to bat for more vaccine cooperation to beat the pandemic.

What has Thailand done?

From a high of about 4,000 daily cases in April, a record for the country at the time, Thailand has seen the number of daily new cases jump to more than 8,000 now. News agency AFP says the Southeast Asian kingdom “is struggling to contain its latest outbreak fuelled by the highly contagious Delta variant, with cases and deaths skyrocketing and the healthcare system stretched thin”.

In search of solutions to stem the tide, the country’s health officials have decided to mix a first dose of the Chinese-made Sinovac vaccine with a second dose of the same Oxford-AstraZeneca shot that is known in India as Covishield to try and achieve a “booster” effect in six weeks instead of 12.

The plan to combine the Sinovac shot, which is an inactivated virus vaccine, with AstraZeneca, a viral vector vaccine, comes as health officials are looking to cut the duration between doses. “We can’t wait 12 weeks (for a booster effect) in this outbreak where the disease is spreading fast,” said the country’s chief virologist, Yong Poovorawan.

However, WHO has warned against such moves with its Chief Scientist Soumya Swaminathan terming any strategy not founded on scientific evidence a “dangerous trend”.

“We are in a bit of a data-free, evidence-free zone as far as ‘mix-and-match'” is concerned, she said.

According to reports, Swaminathan’s concern stemmed mainly from a lack of data on mixing of the two vaccines in question. “There are studies going on; we need to wait for that and maybe it will be a very good approach but at the moment we only have data on the Oxford AstraZeneca vaccine followed by Pfizer,” Swaminathan said.

There have been multiple studies on combining the Oxford–AstraZeneca and the Pfizer–BioNTech shots that have found that such a move “triggers an immune response similar to — or even stronger than — two doses of either vaccine”.

So while there is a case being built up for mixing vaccines, there is a need for strong data before any such strategy is adopted. Further, shots should be mixed only upon advice of public health officials and individuals should not take such a decision on their own.

“Individuals should not decide for themselves, public health agencies can, based on available data. Data from mix and match studies of different vaccines are awaited – immunogenicity and safety both need to be evaluated,” Swaminathan said in a tweet on 13 June.