Can a vaccinated person still spread coronavirus?

HEALTH AFFAIRS: Several Israeli research teams are getting ‘very promising’ results that the answer is no.

PEOPLE WAIT to receive their COVID-19 vaccine injections outside a mobile Magen David Station at the Mahaneh Yehuda market in Jerusalem, on Monday. (photo credit: OLIVIER FITOUSSI/FLASH90)
PEOPLE WAIT to receive their COVID-19 vaccine injections outside a mobile Magen David Station at the Mahaneh Yehuda market in Jerusalem, on Monday.
(photo credit: OLIVIER FITOUSSI/FLASH90)
 After nearly two months, Israeli data is beginning to confirm what Pfizer already knew: their coronavirus vaccine stops symptomatic and severe COVID-19.
So, why are Israelis still wearing masks for anything but Purim?
Health experts are still unsure whether the vaccines prevent asymptomatic cases – contracting the virus but having no symptoms. If those people who get vaccinated can still contract coronavirus and transmit it to others, then it will be difficult to stop the spread of COVID-19. 
If people silently become infected with coronavirus, they could pass it on, potentially sickening people who are not immune. 
A model developed by the researchers at the Centers for Disease Control and Prevention in January showed that people with no symptoms transmit more than half of all cases of the novel coronavirus.
But there is growing evidence that people who get vaccinated do not spread the virus very much, if at all.
The ideal vaccine is one that produces what is known as sterilizing immunity, which means that your immune system is able to stop a pathogen, including viruses, from replicating within your body. Not all vaccines achieve this standard. For example, the measles and rubella vaccines provide it. The hepatitis B vaccine does not.
Although it is scientifically intuitive that a reduction in infection and transmission would come with vaccination, and preliminary signs suggest that the vaccine does do at least some of both, according to Cyrille Cohen, head of the immunotherapy laboratory at Bar-Ilan University, “we need proof” and that these are exceedingly difficult studies to do.
Having the answer would have “obvious” and “major implications” on people’s daily lives, said Eran Segal, a computational biologist at the Weizmann Institute of Science. That is because to achieve herd immunity, the vaccine would need to prevent transmission.
The more the virus circulates, he explained, the more opportunity it has to mutate in ways that enhance its ability to spread – like the British variant – to make people sick or become vaccine resistant.
“If you have people coming from abroad and you know there is a small chance they will pass on the virus, we can let them go and they do not have to quarantine,” Segal explained. “But if you know there is a non-negligible chance that they can still pass on the virus, then you have to pay attention, because of the variants.”
He said it also relates to the general opening up of society, such as the ability to hold large concerts without concern. Or, to hold those concerts without social distancing and masks.
“Everyone still needs to be cautious,” said Prof. Gili Regev-Yochay, director of the Infectious Disease Epidemiology Unit at Sheba Medical Center, Tel Hashomer. “Things are more complicated now because people think they are vaccinated and everything is good, so it’s harder to get them to keep on their masks. But it is necessary because we do know that some people will get infected and be asymptomatic. And those people may be able to spread the disease.
“The problem is that if not everyone is vaccinated or there are people who are immunosuppressed, even if they are vaccinated the vaccine is less effective, they may get the disease,” she continued.
REGEV-YOCHAY said several Israeli research teams are now looking into the question of transmission after coronavirus vaccination and “the results are looking very promising.”
Moderna showed early positive results that people who get infected were less likely to transmit the disease in its original reporting, which was submitted to the US Food and Drug Administration in December when it achieved approval for emergency use.
During its Phase III clinical trial, Moderna researchers swabbed participants to see if they had the virus when they went for their second shot and then compared the results of those who received the vaccine versus the placebo.
They saw a two-thirds drop in the number of asymptomatic infections among people who received the first shot compared to those who received the placebo.
In a supplement provided to the FDA, it showed that 14 of the 14,134 vaccinated people had asymptomatic cases of coronavirus at the time, versus 38 of the 14,073 in the control group.
However, this was not the primary endpoint of the Moderna research. They only tested people twice – at the beginning and before the second shot, making the screenings about a month apart – so they may have missed infection.
Oxford University in its clinical trial for AstraZeneca also screened participants for the virus. Different reports by the university showed between a 49% and a 67% reduction in positive tests among the vaccinated compared to the unvaccinated.
Earlier this week, a SIREN (SARS-COV2 immunity and reinfection evaluation) report by Public Health England was preprinted online by the peer-reviewed Lancet medical journal that evaluated staff working in publicly funded hospitals in the UK.
The study measured the impact of a single dose of the Pfizer vaccine over an eight-week period and found “strong evidence that vaccinating working-age adults will substantially reduce asymptomatic and symptomatic SARS-CoV-2 infection and therefore reduce transmission of infection in the population.”
Specifically, the report showed that there were 977 new infections during 710,587 person-days of follow-up in the unvaccinated group, an incidence density of 14 infections per 10,000 person-days. In the vaccinated group, 21 days after the first dose, there were 71 new infections, an incident rate of eight per 10,000 person-days of follow up, and nine new infections seven days after the second dose, an incident rate of four per 10,000 person-days of follow up. Person-time, as described by the North Carolina Institute for Public Health, is an estimate of the actual time-at-risk that all participants contributed to a study.
“After controlling for other risk factors... vaccine effectiveness against infection 21 days after the first dose of the Pfizer vaccine in the overall study population was 70% and increased to 85% seven days after the second dose.”
Staff was PCR tested regardless of symptoms in order to detect asymptomatic infection.
In Israel, where nearly 4.6 million people have received at least one jab, data is starting to emerge, as well.
A prepublished study by the Health Ministry and Pfizer that was released through the media showed the vaccine reduces infection, including asymptomatic infection, by 89.4% in people who got two doses of the vaccine compared to unvaccinated people.
However, as noted, the full report has not yet been released and some scientists have said that the report’s methodology may have led to an overestimation of the vaccine’s effect.
Separately, a report by Sheba that was released in the Lancet earlier this month found that among more than 7,000 healthcare workers who received their first dose of the Pfizer vaccine in January, there was a 75% reduction in coronavirus cases within 15 to 28 days.
The hospital’s Regev-Yochay said only 170 people became infected during the two-week period. Of those who contracted the virus, 99 showed symptoms. Eighty-nine of the sick were unvaccinated.
THERE IS also another consideration when it comes to virus transmission, and that is viral load: How much virus can be measured in a patient, which will determine how much virus you are spreading into the air when you breathe or cough. The less virus you spread, the fewer people who are likely to contract the virus from you.
The effect of vaccination on viral loads in coronavirus post-vaccination infections is still unknown, but some studies are starting to surface on that subject too.
A paper published earlier this month on Medrxiv, a non-peer-reviewed health sciences website, by researchers from the Technion-Israel Institute of Technology and Maccabi Health Services, reported that mean viral load substantially decreased 12 days post-vaccination.
“Analyzing positive SARS-CoV-2 test results following inoculation with the BNT162b2 mRNA vaccine [the Pfizer vaccine], we find that the viral load is reduced four-fold for infections occurring 12-28 days after the first dose of vaccine,” the report said. “These reduced viral loads hint to lower infectiousness, further contributing to vaccine impact on virus spread.”
The report was based on an observational study, not a randomized, clinical trial. As such, it had several limitations.
Also, this month, one of the country’s largest testing labs, MyHeritage, published a study comparing the viral load of some 16,000 positive test results between December 1 and January 30. The study was hoping to evaluate if there were any differences between the amount of virus present in people over the age of 60 – the first people in the country to be vaccinated – and those between the ages of 40 and 59, who started vaccinating later.
The lab, together with researchers from some of Israel’s top universities, considered four two-week-long windows, the first three before most of the older population had been vaccinated.
Israel kicked off its vaccination campaign on December 20 with medical workers and the elderly, so by the end of the testing period, around 80% of people over 60 in Israel had received at least one vaccine compared to around 30% or 40% of those ages 40 to 59.
According to the study, in the last two weeks, those over 60 presented a viral load lower by 50% to 95%.
However, here too, there were limitations, such as that the researchers did not know if the people being tested for coronavirus were vaccinated at all, let alone with one or two shots.
Bar-Ilan’s Cohen said there has also been anecdotal evidence surfacing in the country as more people get inoculated.
“We know, for example, that there was an anecdote that two healthcare people who were vaccinated became infected by the virus but no one in their immediate surroundings at home or work got the virus from these people,” Cohen said.
“Somehow the vaccine was protective,” he continued. “But this is only an anecdote. In science, we need real proof, and it is very difficult to do such studies.”
As noted, the original Pfizer, Moderna or AstraZeneca trials were not set up to answer this question. Rather, they were meant to determine the safety and efficacy of their vaccine candidates.
Normally, in these types of clinical trials, volunteers are divided into two groups – one that receives the vaccine candidate and the other a placebo. Once the number reaches a critical, preset trial mass, the researchers compare the two groups to see if the ones who got the authentic jab fared better.
Pfizer and Moderna reported more than 90% efficacy, and the side-effects were considered mild and minor.
THE REASON that transmission studies are not done in the first stage is that they take a long time and are difficult to properly conduct. Experts estimate that proof of reduced transmission is at least several weeks if not years off.
“It is not by chance that Moderna and Pfizer did not set prevention of transmission as one of their primary or secondary objectives in clinical studies of the vaccine, because they really take time,” Cohen said. “You have to follow people who were vaccinated for a long period of time and determine if they infected other people.”
For example, he said you may take a family where one person is vaccinated and then test that person and his family members regularly to decide if any person was contaminated by the vaccinee. But Cohen said this is not foolproof either, as family members could also be contaminated by an external source.
Segal said another way of doing it is to track a large cohort of tens of thousands of people from the point at which they get vaccinated, testing them every week to see if they actually got infected.
“This is not going to be over until it is over,” Regev-Yochay said of the pandemic. “We are still not there.”