Are vaccines and boosters necessary even after infection?
Many around the world are asking whether primary jabs or follow-up doses are still needed if a person has been hit with Covid-19. In theory, infection can induce antibodies that may result in so-called natural immunity. But key questions remain. We examine what the regulators and scientific studies tell us.
Does someone who has already been infected need to be vaccinated? If they are vaccinated, should they get boosters?
The answer is yes, and yes. According to the United States Centres for Disease Control and Prevention (CDC), people who have already had Covid-19 and do not get vaccinated after their recovery are more likely to get Covid-19 again than those who get a jab after their recovery.
Since a booster can better protect a person from severe disease, health authorities in many countries advise people to get boosted even after infection, though guidelines vary.
In Singapore, for example, people can receive a booster 28 days after infection, although the recommendation is to be boosted three months after infection. British health authorities are advising at least a four-week wait after infection before getting a booster, while in Hong Kong there is no urgency for recovered patients to have a third dose, although a first dose is advised.
A study funded by the US CDC released in November last year showed unvaccinated adults admitted to hospital with Covid-19 were 5.49 times more likely to be reinfected 90 to 179 days later than those who had received two doses of the Pfizer/BioNTech vaccine but were never infected.
The UK Health Security Agency said last month a study it sponsored showed that unvaccinated participants who had been infected with Covid-19 were found to have 86 per cent lower risk of reinfection compared to the risk of primary infection in those who had no previous infection and were also unvaccinated. But the protection wanes over time.
Double protection in people who were both infected and then double vaccinated was greater and more long lasting, standing at over 90 per cent after two doses. This protection remained strong more than a year after infection and more than six months following vaccination, it said.
How long will naturally acquired protection last? And when should a person receive the first vaccine doses or a booster after recovering from infection?
Studies show that, in general, natural infection can protect for around six months, with immunity starting to wane after four to six months. A study by Johns Hopkins University showed people who had two mRNA vaccine doses but were infected later had longer protection than those who were infected but unvaccinated.
“It is important to remember that some people with antibodies to Sars-CoV-2 may become infected after vaccination [vaccine breakthrough infection] or after recovering from a past infection [reinfected]. Based on what we know right now, risk of reinfection is low for at least the first six months following an infection with the virus that causes Covid-19 diagnosed by a laboratory test,” the US CDC said.
In Hong Kong, the government is carrying out mass inoculation in homes for the elderly as the city is hit by a severe fifth wave. Unvaccinated elderly residents who have recovered from Covid-19 receive a vaccine shot one month after recovery while those who had been inoculated receive a dose three months after recovering from Covid-19.
If a person is infected by other variants and has recovered, will they be adequately protected against the Omicron variant?
Experience in South Africa showed previous infection by other variants did little to prevent an Omicron infection. Also, a study in Qatar published in The New England Journal of Medicine last month showed previous infection was estimated to be 90.2 per cent effective in protecting against the Alpha variant, 85.7 per cent effective against the Beta variant and 92.0 per cent effective against Delta – but just 56.0 per cent effective against Omicron.
Health authorities in many countries advise people to get boosted even after infection