It’s still unclear how long the vaccines or recovery of COVID-19 will protect us against reinfection and whether it will shield us against new variants. Some experts suggest that the COVID-19 booster shots to bolster immunity are inevitable, but it’s tough to know when we should begin.
Most of us have experience with booster shots for other infections. Typically, they’re necessary when the antibodies in the blood fall below a protective threshold. For example, public health authorities recommend tetanus, diphtheria, and pertussis vaccines at ten-year intervals when antibody titers generally decline. For hepatitis A, it’s necessary to get a booster shot one year after the first dose.
We also need booster vaccinations because of the emergence of new viral variants that make the previous vaccine protection less effective – that’s why we need annual flu shots.
Some vaccines like the childhood vaccines for measles, mumps, and rubella don’t require boosters because they usually generate lifelong immunity. These vaccines contain live replicating viruses that tend to provide more extended protection.
Lessons from veterinary coronavirus
Veterinarians have been treating coronavirus infections in livestock and pets for many years. Examples include avian infectious bronchitis, porcine transmissible gastroenteritis, and feline infectious peritonitis. Their experience with these infections and vaccination strategies provides insights into the current COVID-19 pandemic. For instance, they’ve used many different vaccination approaches such as oral, intranasal, or intramuscular routes with killed/inactivated or live/attenuated whole virus vaccines with varying success.
Veterinary strategies to ensure long-lasting protection have included combining vaccines or using multiple virus strains within one vaccine. Other trials included inoculating with an oral dose followed by an intramuscular vaccine injection or using one vaccine strain and then boosting with a different strain. The main aim has been to broaden the host’s defense by promoting long-lasting protection with neutralizing antibodies and even cytotoxic killer lymphocytes. Vaccines that elicit both antibody responses and cell-mediated immune responses seem to improve protective immunity and viral clearance.
The duration of protective immunity in vaccinated animals is dependent on the type of vaccine, animal, and virus. However, protection generally seems to last about a year which means that the animals need an annual booster, though some vaccines require more frequent and even multiple boosters. Unfortunately, there’s little or no experience with RNA coronavirus vaccines like Pfizer and Moderna in animals to provide us with the crucial data we need for this COVID-19 vaccine approach.
A COVID-19 booster is likely
For COVID-19, we need to establish the “booster threshold”, which means the number of fully vaccinated people who develop severe infection requiring hospitalization. We also need to figure out the threshold antibody levels, how long protection after vaccination lasts, and the rate of viral mutations.
Researchers are developing predictive models that simulate COVID-19 vaccine protection based on the concentration of the protective antibodies, how long they stay elevated, how fast they decrease, and correlate these results with protection from severe disease.
But to get a better idea of whether we need boosters and when we’d need them, we must firmly establish the time that it takes for protective immunity to wane in a fully vaccinated person and at what point they become contagious or severely ill when re-infected with SARS-CoV-2. We’ll also need to consider booster shots to combat viral variants that escape the current vaccines – antibodies that don’t recognize the viral mutations.
In Israel, they have already begun to provide booster shots to at-risk people.
Be prepared! We will most likely need annual COVID-19 boosters, and they may be needed as early as 6-12 months following our second vaccine. And with them, we can continue our active lives.