Can vaccines stop the spread of coronavirus? – POLITICO.eu
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For all the relief brought on by the arrival of the first coronavirus jabs, one big question looms: Will they also prevent transmission?
While clinical trials of the first vaccines showed success in preventing the disease from developing, scientists have yet to establish whether vaccinated people — without symptoms — can spread it.
The question goes to the heart of a growing debate over vaccine passports — favored in particular by tourism-dependent European countries such as Greece — that would allow vaccinated people the freedom to travel. Policymakers are so keen to better understand this issue that EU leaders will convene this Thursday to discuss it.
Even in countries which have made most progress on vaccinations, it is too early to answer the transmission question. In Israel, a leader in vaccination rates, there were positive signs last week. With roughly a quarter of the population now vaccinated, studies show it is preventing disease in up to 60 percent of recipients after the first jab. Doctors hope that fewer symptomatic cases (for example, no coughing) might lead to lower transmission.
But those hopes have yet to translate into lower infection rates overall. The latest data, reported Monday, showed the highest level on infection rates to date.
Meanwhile, in the U.K., which leads vaccine distribution in Europe, experts are staying cautious.
Mary Ramsay, head of immunization at Public Health England, said during questioning by lawmakers last week that she thinks it’s “very likely” that the freshly approved vaccines block (or “sterilize”) transmission to some extent, but cautioned that spread was still likely.
Sarah Caddy, a research fellow at the University of Cambridge, wrote in an op-ed that “in an ideal world, all vaccines would induce sterilizing immunity” — but “in reality, it is actually extremely difficult to produce vaccines that stop virus infection altogether.”
Experience with earlier vaccines has certainly shown that some can be designed to block transmission. But scientists point out these may have difficult side effects, or just not be effective in the broader fight against a virus.
In the long run, some scientists suggest that the emphasis on blocking transmission is misplaced. An alternative way to reduce the disease’s severity, they say, is to allow ongoing low-level transmission among both vaccinated and unvaccinated people to keep the immune system primed for future infections.
This would mean readjusting expectations and accepting that the virus is here to stay rather than viewing vaccine effectiveness in the context of transmission.
“I doubt that we will be able to stop transmission, either by vaccination or natural immunity,” said Paul Hunter, professor in medicine at Norwich School of Medicine. “[The virus] will continue to spread for decades to come.”
But this outcome “may not be a bad thing,” he added. “Repeat infections every few years may be what prevents us getting a severe disease in 10 years’ time rather than big gaps between mini epidemics.”
Still, in the short term, the world is desperate to get back to normal. To that end, some researchers say the best way to quickly reduce transmission and protect the vulnerable is to change priorities so that active, working-age people — who are more likely to spread the virus — get the jab first.
The immune system is a clever and complicated thing, constantly clearing out viruses and bacteria that would otherwise make us sick.
Natural infection with coronavirus triggers a very broad immune response via the production of antibodies and T cells. This includes antibodies in mucosal membranes, which are found in the nose, mouth and throat. When exposed to the virus a second time, these antibodies help block it from causing severe disease by killing it off before it filters down into the body.
Scientists believe that this “mopping up” of the virus in the nose, mouth and throat will help prevent its spread via droplets that are exhaled, coughed or sneezed out.
The question that vaccine manufacturers haven’t yet studied, however, is whether the new vaccines produce these antibodies in the mucosal membranes.
The approved vaccines, and most in development, focus on one part of the virus — the spike protein — because it has been shown to provoke a strong immune response. But it’s just one among the 20 to 30 different proteins on the virus that aren’t targeted, according to virologist Julian Tang of the University of Leicester.
Some vaccines are using a version of the whole coronavirus, including those from Chinese biotechs Sinovac and Sinopharm. These may indeed provoke a broader immune response, including producing antibodies in the mucosal membrane — meaning they could also prevent transmission. On the downside, according to Tang, these usually bring an increased risk of disease-like side effects — an example being some flu shots in the 1960s.
“They produce a good immune response but give you a lot of adverse effects,” he said, noting they were eventually discontinued because “people stopped taking them.” This yielded a new problem: Once people stopped getting vaccinated, they were spreading the virus to vulnerable people.
Tang notes that the Chinese haven’t released much data on adverse effects. For now, their reports simply say side effects are “very low.” French biotech Valneva is also using a whole inactivated virus approach — the only one in this class undergoing clinical trials in Europe.
However, some next-generation coronavirus vaccines — which would be delivered nasally to induce an immune response at the level of the mucosal membranes — could offer extra gains “in terms of local protection at the site of infection,” according to Danny Altmann, professor of immunology at Imperial College London and the British Society for Immunology.
Some vaccines in development are already showing signs of inducing sterilizing immunity, he points out. This includes Novavax’s protein-based vaccine as well as Sinovac’s traditional whole virus technology, noted above. This is not because they target localized immune responses in the nose and throat, Altmann believes, but that they “induce really brilliant levels” of antibodies throughout the body.
Some scientists think the fixation on ending transmission entirely misses the mark. Tang, for one, calls it a “red herring.”
As he points out, there’s a reason why scientists usually don’t follow up on transmission rates after vaccination: Once an entire population is vaccinated, it doesn’t actually matter if they’re infecting others because most will already be immune — transmission “hits a brick wall.”
So while immunized people may still become infected, their body is primed to fight off the virus and prevent severe disease. While infected, however, they still could be spreading the virus without any symptoms.
Altmann thinks people have become preoccupied with “sterilizing vaccines,” since if a jab is so effective that it eliminates the virus in the nose and throat, there’s a chance of wiping it out rather than letting it “percolate” at a low levels in the population.
Tang believes a more effective short-term route is to focus on vaccinating those most likely to spread the virus, like the young and mobile —an approach already underway in Indonesia. Its population is relatively young, and therefore less at risk for severe COVID-19 (only 5 percent of Indonesians are over 65, versus 18 percent in the U.K.) So in terms of blocking transmission, the government’s policy of shielding the elderly while vaccinating the younger population makes more sense, Tang says.
“You should vaccinate postmen, teachers, people who work in supermarkets,” he said. “That’s where transmission is happening.”
He notes that each year, children in the U.K. are given a live flu virus vaccine up the nose, which mimics a natural infection of influenza. “That protects their parents, who are more vulnerable,” he said.
There is also the question of whether we truly need to develop vaccines that prevent transmission completely. As Tang pointed out, lower levels of virus in a person (the “viral load”) typically lead to less severe disease — so if you can kill off most of the virus after being vaccinated but still spread it at low levels, it will be less debilitating despite continuing to circulate.
“We’ve become very hung up” on vaccines that try to wipe out the virus in the nose, mouth and throat, Altmann said. “But what if [the virus stays] at a low level that makes transmission much less likely?”
Evidence on its way
For now, Altmann notes, the early news from Israel shows the BioNTech/Pfizer mRNA vaccine “is already driving transmission right down.”
With vaccines now widely rolled out in the U.K. — 4 million doses have now been administered — researchers are starting to study how well they stop the spread of the virus.
In its vaccine surveillance strategy, Public Health England (PHE) said understanding whether vaccines reduce transmission “will inform policy decisions around whether to prioritize vaccinating those with worse outcomes or those who transmit more.”
Two studies are already underway to investigate whether vaccination reduces the risk of transmission in health care workers and care home staff, two frequently tested groups. There’s also a study checking for symptomatic and asymptomatic secondary cases in households, the PHE plan notes, comparing secondary infection rates where the primary case has been vaccinated to those where the primary case didn’t receive the jab.
Despite all the unanswered questions, Altmann describes himself as “moderately optimistic” that vaccines will have a positive effect on reducing transmission and points to the PHE effort as an important signpost.
The results will “come out in the wash fairly rapidly,” he said.
Ashleigh Furlong contributed reporting.
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