Covid-19 vaccine now available to 12–17 age group – but will just one shot be effective?

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  • Covid vaccinations have officially opened up for 12-17-year-olds in South Africa.
  • However, only one dose of Pfizer's two-dose Covid vaccine series will be offered for now.
  • Three experts weigh in on whether just one shot will offer effective protection against Covid, as well as reduce transmission.

Covid-19 vaccinations officially opened to children 12 years and older on 20 October. Increasing South Africa’s vaccination drive to include teens will add another six million people to the programme, health minister Joe Phaahla recently said.

But this age group is eligible for just one Pfizer shot – the vaccine requires two doses given at least 21 days apart to achieve full protection. Pfizer’s vaccine is currently the only Covid jab that has received approval from the South African Health Products Regulatory Authority (Sahpra) for use in people 12 years and older.

The decision to administer one dose is based on reported cases of myocarditis (inflammation of the heart muscle) that occurred internationally, mainly after the second dose. It has mostly been observed in male adolescents and young adults, although cases are rare and have been generally mild and short-lived. 

Preventing onward transmission

South Africa’s ministerial advisory committee on Covid vaccines (VMAC) is currently awaiting additional safety data on this potential side effect within the group, said Phaahla.

Although teens are less likely than adults to fall seriously ill because of Covid, some of them who are infected can get severe lung infections, become very ill and require hospitalisation, Johns Hopkins Medicine notes

There’s another, more impactful effect of vaccinating this group: it will help reduce transmission of the virus among the population. 

“Children are at low risk of severe disease to start with. The motivation for vaccinating children is not only protection from infection, severe disease or death, but also for  the ability of the vaccine to prevent onward transmission of  the virus to others,” senior epidemiologist at the National Institute for Communicable Diseases (NICD) for HIV and STIs, Dr Tendesayi Kufa-Chakezha, told Health24.

Data by the US Centers for Disease Control and Prevention (CDC) points to outbreaks among children, particularly at camps or schools, proving that younger people can contract the infection and spread the virus. Research shows that those who are vaccinated have a lower viral load on average than unvaccinated people, proving that vaccinated individuals are less likely to spread Covid.

But how effective will one dose of the Pfizer vaccine be in protecting 12–17-year-olds, and can it work to limit transmission of the virus? We asked the experts.

'Unlikely to have public health impact'

In a webinar hosted by My Health 360 on 15 October, Professor Shabir Madhi, Dean of the Faculty of Health Sciences and Professor of Vaccinology at the University of the Witwatersrand, said he was not convinced by the government’s decision to open up vaccinations to 12–17-year-olds.

Like Kufa-Chakezha, Madhi highlighted that this would mainly serve to provide indirect protection to the population. “But that is not going to assist much if we have a significant percentage of people above the age of 50 that are unvaccinated,” which is the case in South Africa, he said.

Speaking to Health24, Madhi said that a single dose of Pfizer (and most other vaccines) works quite well in protecting against severe Covid disease, but protects only modestly against infection and mild Covid, especially with infection with the Delta (and Beta) variant, which is currently dominating infections in the country. 

“As such, a single dose is unlikely to have much of a public health impact, nor will it confer much individual protection in the context of the average 12–17-year-old who does not have underlying risk conditions for developing severe Covid,” he said.

Reducing transmission: one vs two doses

Kufa-Chakezha commented that, in the clinical trial, which had a very short duration of follow-up (a median follow-up of around two months after the second dose), and where most children 12–15 years old received two doses, these two doses reduced new Covid  cases by 100%, while one dose reduced it by 75%. 

“Because there is no long-term data on children, we may have to look at studies in adults. Other studies of healthcare workers are showing that two doses are better than one in reducing infection in the healthcare worker and also transmission to contacts,” she said.

Kufa-Chakezha added that two doses reduce this transmission risk by 70–90%, compared to 40–50% for one dose. “One dose would help, but not as much as two,” she said.

The extent to which the vaccination will prevent onward transmission of the virus will also depend on coverage in both adults and the children themselves, said Kufa-Chakezha, where high vaccination coverage will translate to greater effectiveness. 

“This invitation to vaccinate children should encourage households and families to discuss Covid-19 vaccination and promote greater access to both adults and children,” she said.

'Two doses would be much better'

Professor Heather Zar, a leading public sector  paediatric pulmonologist at Cape Town's Red Cross Children's Hospital, believes it’s great that the government has opened up vaccinations to 12–17-year-olds, especially as they have been more affected with the Delta variant in the third wave compared to the numbers of adolescents infected or sick in the first and second waves.

“However I think one Pfizer dose is inadequate protection – two doses would be much better,” she said, adding: “We know that one Pfizer dose is protective against severe disease (but adolescents are very unlikely to get severe disease anyway), but doesn’t provide good protection against infection (or asymptomatic or mild illness).  

One of the key reasons to vaccinate adolescents is to prevent transmission of the virus to others in the community or vulnerable people, such as school staff, in households, and public spaces. Therefore, there is a strong public health reason to vaccinate adolescents, said Zar. 

“But a single Pfizer dose does not provide this – two doses are needed,” she stressed. “We know from accumulating data from the UK, US, Israel, and other countries that two mRNA doses provide substantial protection against infection including against the Delta virus.”

One dose could still be effective

If the reason for administering just the first dose was really about the risk of myocarditis after the second dose, then this decision should only apply to boys in whom this risk has been recognised, and not so in girls, said Madhi. 

“The one issue that lends some credence to a single dose – not only in this age group but also other age groups – is that if the majority of South Africans have been previously infected with the virus, then a single dose will accomplish as much (if not more) than two doses in people who had not been previously infected with the virus,” he said.

Recently calculations by Discovery Health indicated that 80% of South Africans may already have contracted the virus during the past three waves.

*For more Covid-19 research, science and news, click here. You can also sign up for our Daily Dose newsletter here.

READ | Covid-19: SA must vaccinate more adults above 50 years before fourth wave – Prof Madhi

READ | SA teens are in line to be vaccinated: What you need to know

READ | What to know about the mRNA Covid vaccines and risk of heart inflammation in children

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