WASHINGTON (BLOOMBERG) - With hundreds of millions of people now vaccinated against Covid-19, the coronavirus outbreak should begin to die down in places where a large chunk of the population has been inoculated. But that is not happening everywhere.

Instead, two paths are emerging: In countries such as Israel, new Covid-19 cases are declining as more people are vaccinated, while in other places like the Seychelles - which has fully inoculated more of its population than any other nation - infections continue to increase or even reach new highs.

One reason for that may be the different types of vaccine being used.

Evidence derived from the expanding global inoculation roll-out indicates that the messenger RNA (mRNA) shots developed by Pfizer-BioNTech or Moderna are better at stopping people from becoming contagious, helping reduce onward transmission - an unexpected extra benefit as the first wave of Covid-19 vaccines were intended to stop people from becoming very sick.

Other vaccines, while effective in preventing acute illness or death from Covid-19, appear not to have this extra perk to the same degree.

"This will be an increasing trend as countries start to realise that some vaccines are better than others," said Professor Nikolai Petrovsky from the College of Medicine and Public Health at Flinders University in South Australia.

While the use of any vaccine "is still better than nothing", he said, some doses "may have little benefit in preventing spread, even if they reduce the risk of death or severe disease".

Studies of millions of people in Israel vaccinated with the Pfizer-BioNTech shot show that the mRNA doses prevented more than 90 per cent of asymptomatic infections - those who contract the virus but show no symptoms.

That is important, said Dr Raina MacIntyre, an epidemiologist at the University of New South Wales in Sydney, because a vaccine's ability to stop asymptomatic infection "is the determinant of whether or not herd immunity is possible".

Herd immunity is typically achieved when the virus can no longer find any vulnerable hosts in order to keep spreading.

Which vaccines a country can secure could, therefore, affect everything from policy about mask-wearing and social distancing to lifting border restrictions and reviving economies, given the influence that daily case counts have on government decisions. For individuals, it may mean how soon they regain pre-pandemic freedoms.

In the United States, nearly 40 per cent of the population has been fully vaccinated, mostly with mRNA shots, and the number of new cases each day has dropped by more than 85 per cent in the past four months.

A similar trajectory is evident in Israel, which fully vaccinated nearly 60 per cent of its population with the Pfizer-BioNTech shot and has gradually lifted restrictions as new cases declined to fewer than 50 a day, from more than 8,000 at the beginning of this year.

Qatar and Malta are also witnessing a decline in new cases after giving around 30 per cent of their populations two doses of mostly mRNA vaccines.

The results provide further evidence of the surprise efficacy of the new mRNA shots; the pandemic is the first time this vaccine technology is being widely used. They work by delivering genetic codes that instruct the human body to make proteins of the virus that in turn stimulate an immune response.

The existing mRNA shots require ultra-cold storage, limiting their accessibility to countries with poor transport and storage infrastructure.

That is one reason why most countries, though, are relying predominantly on non-mRNA shots ranging from the adenovirus vector vaccines from AstraZeneca to those from Chinese developers, Sinopharm and Sinovac Biotech, that use an inactivated form of the virus. These more traditional vaccine types have shown efficacy rates of between 50 per cent and 80 per cent in preventing symptomatic Covid-19 in clinical trials, compared with more than 90 per cent for mRNA ones.

There is little data regarding their ability to prevent onward transmission, but signs are emerging that it may be much lower.

AstraZeneca pointed to the success of Britain's campaign as evidence of its shot's benefits, though the results include the impact of Pfizer-BioNTech's mRNA inoculation. Covid-19 infections in Britain fell by 65 per cent after a first dose of either shot, while household transmissions dropped by as much as 50 per cent.

Sinopharm did not immediately respond to queries for this story.

Mild symptoms

An archipelago in the Indian Ocean, the Seychelles has fully vaccinated about 65 per cent of its population with AstraZeneca and Sinopharm shots, yet weekly new infections increased rapidly this month, with 37 per cent of those patients having already received their two doses. The surge led the authorities to close schools, cancel sporting events and ban household gatherings.

Among fully inoculation people, around 60 per cent received the vaccine from Sinopharm and the rest got AstraZeneca's shot.

In Chile in South America, the steady ramp-up in vaccination using mostly Sinovac's shot did not prevent the number of new daily cases from almost doubling in mid-April from a month earlier, despite administering enough doses to cover 30 per cent of the population. The authorities had to reintroduce a lockdown across the country in late March.

But in the case of both countries, the type of vaccine available seems to be only one of many factors affecting the spread of the disease.

Policy failures in places that reacted too slowly to impose restrictions, or lifted them prematurely, have been a major contributor, as has the speed of diagnosis and access to medical facilities. Different transmission rates of virus variants also play a part.

Seychelles President Wavel Ramkalawan said transmission increased in his country after the nation's world-beating vaccine roll-out caused people to let down their guard.

The uptick in infections in Chile was attributed by Sinovac's chief executive Yin Weidong to the nation prioritising elderly people in its initial vaccination efforts.

"It's normal that the country sees a resurgence of infections as social activities increase among the younger people who are mainly not inoculated," he said in a May 11 interview.

Data for Pfizer-BioNTech and Sinovac vaccines are from real-world studies in Israel and Chile respectively, while that for AstraZeneca is from its clinical trials.

What seems clear is that all the approved doses reduce the incidence of people becoming severely ill or dying from the disease - the primary goal of a vaccine. That takes pressure off hospitals and medical resources.

Most new Covid-19 patients in the Seychelles, for example, are only experiencing mild symptoms, according to the nation's president.

This is a crucial first step for countries without access to mRNA vaccines, said Associate Professor Helen Petousis-Harris, a vaccinologist at the University of Auckland. After using available vaccines to crush the number of severe cases, countries can stamp out remaining infection with shots that curb transmission once they become available.

Another possibility is that vaccination along with the spread of mild cases could also end up building so-called herd immunity, said Professor Ben Cowling, head of the University of Hong Kong's department of epidemiology and biostatistics. "The places with less immunity against infection but more immunity against severe disease will see circulation of the virus causing mainly mild infections, boosting immunity to a higher level."

Experts say a lot remains unknown and so-called breakthrough infections - where people who received shots still get Covid-19 - do occur, even with mRNA vaccines. It is also not yet fully known how well the different kinds of shots perform against the different variants of the virus.

The improvements in the US and Israel came before those places were affected by concerning new strains like those that emerged in Brazil and India.

"I would be careful about over-interpreting the data," said Flinders University's Prof Petrovsky, who pointed out that no head-to-head studies - where shots are tested against each other - have been performed to identify the best vaccines. For the mRNA shots, "data on their effects on transmission is very limited".

In the end, it may need the development of new, modified versions of the vaccines to finally win the war against Covid-19, said Prof Petousis-Harris.

Some vaccine developers are working on nasal spray inoculations, which can prevent the virus from taking hold in the respiratory track, thus cutting off infection at its entry point.

"We've got some super vaccines that are beyond expectation," she said. "We've learnt a lot and so imagine what the next one is going to look like."