So you've gone on and had your COVID-19 vaccine. Great! You're pretty much carefree for the next however-much-immunity-lasts, right? Well... not really. The odds are, you could still spread COVID-19 even as you may not suffer from it yourself because most vaccines don't prevent infection -- they prevent disease.
Two types of immunity
Vaccines essentially work by training your immune system to recognize a pathogen and defend against it. But this doesn't necessarily mean that you're completely immune to the virus. The vaccine essentially prevents symptomatic disease (you won't get "sick"), but whether or not it presents asymptomatic disease is a different question altogether -- and we don't really know the answer to it yet.
Most vaccines don't fully protect against infection, so you can get it and spread it, even though you may be completely asymptomatic yourself and not suffer at all. Vaccinated people can carry the virus to others, and sometimes, they can even start epidemics themselves.
There are two types of immunity you can achieve with vaccines. The first one is "sterilizing immunity", which virtually eliminates infections entirely, preventing a person from getting infected. In an ideal world, all vaccines would produce sterilizing immunity -- but most don't. Most vaccines in use today offer "effective immunity". For instance, you can become infected with pertussis, hepatitis B, mumps, and (often) influenza, even after you've been vaccinated -- but you'd be completely (or almost completely asymptomatic).
This is not a real problem most of the time. For instance, in the US, the incidence of rotavirus-associated hospital visits has decreased by 90% since the vaccine was introduced in 2006, even though it only offers effective immunity. A similar situation is with the polio vaccine, and the virus is almost eradicated worldwide. It's also noteworthy that flu vaccines are extremely apt at controlling the virus despite not offering sterilizing antibodies.
"While sterilising immunity is often the ultimate goal of vaccine design, it is rarely achieved," writes Sarah L Caddy, Clinical Research Fellow in Viral Immunology and Veterinary Surgeon at the University of Cambridge, for The Conversation. "Fortunately, this hasn’t stopped many different vaccines substantially reducing the number of cases of virus infections in the past. By reducing disease levels in individuals, this also reduces virus spread through populations, and this will hopefully bring the current pandemic under control."
Do COVID-19 vaccines provide sterilizing immunity?
Sterilizing immunity happens when the immune response completely blocks infection, including asymptomatic infection. We're not really sure if COVID-19 vaccines offer this.
Effective immunity is produced by a combination of white blood cells and antibodies. To achieve sterilizing immunity, the body must produce something known as "neutralizing antibodies". What we've seen so far from the vaccines is a great deal of success in regards to the former. When the likes of Pfizer and Moderna say their vaccines are 95% effective at preventing disease, they're essentially saying the vaccine provides 95% effective immunity.
This doesn't necessarily mean that they also don't provide sterilizing immunity, but we don't know yet. Companies say that research is currently ongoing to answer the question, but as of now, there's just not enough information out there to draw a definite conclusion. To make matters even more complicated, there's also the effect of confounding factors such as lockdowns. Is vaccination driving the numbers down or is it the lockdowns?
If we look at previous research not related to vaccines, asymptomatic transmission of the virus seems to be responsible for roughly a quarter of infections, so that could also be a significant concern, suggesting that vaccinated people could also be spreading the diseases (though likely, to a lesser extent).
But we have some indirect information that seems somewhat optimistic:
- The chief executive of Pfizer, Albert Bourla, said animal studies found that the Pfizer vaccine offers significant protection from transferring the virus. This hasn't yet been proven in humans, but a small Israeli survey found 98% of the people who received two doses of the vaccine had more antibodies than the ones who had been infected with COVID-19, hinting at sterilizing immunity (although this was a survey, not a clinical trial).
- The AstraZeneca vaccine had different results: in a study on rhesus monkeys, vaccinated monkeys were as likely to become infected as the unvaccinated ones (though they were far less likely to become sick), they did have fewer viral particles in their lungs than the unvaccinated group. In humans, however, the situation seems to be different. A recent pre-print that hasn't been peer-reviewed yet found a 67% decrease in infection, suggesting the "potential for a substantial reduction in transmission."
- Moderna also submitted not-yet-peer-reviewed research that showed that even a single injection can reduce infection by 67%, submitting the research in a brief to the FDA.
- Novavax, whose vaccine hasn't yet been approved anywhere in the world, also has potentially good news: the vaccine prevented the viral spread entirely during studies in rhesus macaques. These results put it in an exclusive club of vaccines that are able to prevent asymptomatic transmission completely in other primates – seen as a promising sign, because they have similar respiratory physiology to humans.
These are encouraging finds, though they still need to be replicated in large-scale clinical trials. So far, vaccines were judged based on their ability to prevent disease, not transmission. It's a pragmatic approach that can yield results -- as we've seen with the likes of polio and the flu, effective vaccination can sometimes be enough.
What does this all mean
The distinction between preventing infection and preventing disease (or, between sterilizing immunity and effective immunity) could become a significant one. Despite some encouraging news, we're not sure that vaccines that prevent disease also prevent infection yet. So what does this mean?
For starters, it's not like vaccinated people get a free pass. Most experts advise people who get vaccinated to continue being cautious: wash your hands, wear a mask, and maintain physical distancing just as before. Basically, act as if you're not vaccinated.
Then, the vaccines are not perfect. A 95% immunity is great, but that still means 1 in 20 people who get the vaccine can get moderate or severe infections and pass the disease on. Unfortunately, this will likely have an impact on how long before we achieve herd immunity and return to normal.
All in all, there's still much we've yet to learn, but studies are well underway. Vaccines are still our only way out of this pandemic, and vaccinating as many people as we can is the best way to achieve some level of herd immunity and get some level of normalcy to our lives.