Can I Still Transfer COVID-19 after Vaccination?

In March, Ghana commenced the COVID-19 vaccination exercise, joining other countries across the world. Clinical trials have shown that the COVID-19 vaccines are very effective at preventing severe sickness associated with COVID-19.

Scientists say the vaccines are also very good at preventing the infection of COVID-19, including asymptomatic infection. But do scientists know the extent to which the vaccines reduce transmission of the virus from a vaccinated person to others who are not vaccinated?

In this report, Fact-Check Ghana reviews published documents and what experts say about the matter.

What has research revealed about vaccination and transmission so far?

A study by the US Centres for Disease Control and Prevention (CDC) has revealed the effectiveness of the two mRNA vaccines developed by Pfizer-BioNTech and Moderna-NIAID. During the research, nasal swabs were collected weekly from all participants to detect the presence of any viral genetic material, regardless of whether they had Covid-19 symptoms. They also collected an additional nasal swab and saliva sample if people developed symptoms.

The results indicated that the vaccines were 90 percent effective at blocking infections — symptomatic and asymptomatic in people who had two doses of the vaccine, and 80 percent effective in people who had one dose. That means there was a 90 percent decrease in infections in people who were fully vaccinated compared with a similar unvaccinated group of people.

Some further research suggests that even if a person who has been vaccinated contracts an infection, the virus may be less infectious in this case — at least for certain vaccines. None of the vaccines is 100 percent effective at preventing infections at the moment so even if people don’t get very sick with COVID-19, they may still contract an infection and can potentially transmit the virus to others, researchers say.

Meanwhile, several research groups are measuring the “viral load” — the concentration of coronavirus particles in people who have been vaccinated — to ascertain the degree of transmission of the virus since earlier research found that viral load is a good indicator of infectiousness.

In a study published in late March in Nature Medicine, Israeli researchers found that people who had been vaccinated with the Pfizer-BioNTech vaccine and later contracted an infection had lower viral loads than unvaccinated people who contracted an infection. The authors noted that “the results show infections occurring 12 days or longer after vaccination have significantly reduced viral loads at the time of testing, potentially affecting viral shedding and contagiousness as well as the severity of the disease.”

The research was, however, an observational study, not a randomised controlled trial, so a number of factors could have affected the results and the outcome may also be different for different vaccines.

In addition, while a lower viral load suggests less infectiousness, the researchers say they don’t currently know the “infectious dose” of the coronavirus for people. They say additional studies are needed to determine whether vaccines prevent transmission or not. This includes studies that involve contact tracing to see if family, friends, and other close contacts of vaccinated people are indirectly protected from infection.

After primary analysis of its phase III trials, AstraZeneca, the vaccine which constitutes the greater percentage of doses being administered in Ghana, was confirmed to provide 100% protection against severe disease, hospitalisation, and death through COVID-19 with over 70% protection starting after a first dose. The efficacy is increased with longer inter-dose interval and first indication of reduction in disease transmission of up to 67%.

Relationship between variants and transmission

Scientists are concerned that certain coronavirus variants may reduce the effectiveness of vaccines, which could also affect transmission after vaccination.
This includes the B.1.351 variant first detected in South Africa, the P.1 variant first detected in Brazil, and the B.1.526 variant, which is spreading rapidly in New York.

All these variants contain a mutation called E484K which may help the virus evade antibodies produced by the immune system. This might also make vaccines less effective.

Clinical trials show that the Johnson & Johnson and Novavax vaccines were less effective at preventing symptomatic infection by the B.1.351 variant compared with the original coronavirus. Both vaccines, though, were still effective at reducing the risk of severe COVID-19.

Current COVID-19 vaccines are based on the SARS-CoV-2 spike protein, which the virus uses to bind to and infect host cells, of the original Wuhan-hu-1. But the emerging “variants of concern” deemed so because they appear to be more transmissible or deadlier than the wild-type Sars-Cov-2 which contain mutations in the spike protein, spurring vaccine efficacy concerns.

Trials of the Novavax, Janssen/Johnson & Johnson, and AstraZeneca vaccines in South Africa, where the B.1.351 variant of concern represents virtually all of the circulating SARS-CoV-2, seemed to justify those concerns. The South Africa trials found lower vaccine efficacy compared with trials in other countries where B.1.351 wasn’t dominant.

However, Pfizer’s recently released data showed that its vaccine was highly effective in South Africa, where the B.1.351 variant is common.

Dr. Bob Wachter, chair of the department of medicine at the University of California, San Francisco, pointed out on Twitter that when thinking about the benefits of vaccines, there are two sets.

First, there are individual benefits, such as preventing severe illness, hospitalization, and death.

Second, there are public health benefits: blocking transmission which helps the community by preventing people from transmitting the virus to others, including people most at risk.

Once enough people in the community are vaccinated, the risk of transmission after vaccination becomes less of an issue. Until then, there are other ways to prevent transmitting the virus to others — ones that are available to everyone.

Conclusively, even though scientists haven’t concluded on the relationship between vaccination and transmission of the Coronavirus, some studies have shown that there is a reduced rate of transmission among a community with vaccinated people compared to those without vaccination. Also, the type of vaccine and the variant of the virus come into play in terms of the degree of transmission, with the viral load of the virus playing an important role.

This report is produced under the project: COVID-19 Response in Africa: Together for Reliable Information being implemented with funding support from the European Union.