The impact of natural or induced immunity by future vaccines will be key factors in the future trajectory of the covid-19 pandemic. In particular, a vaccine capable of eliciting a strong immune response could substantially reduce the future burden of infection, according to a study by researchers from the Princeton University (USA) published in the magazine «Science».
“Much of the debate with the future trajectory of Covid-19 has focused on the effects of seasonality and non-pharmaceutical interventions, such as the use of masks and physical distancing,” says researcher Chadi Saad-Roy. “In the short term, and during the pandemic phase, these interventions are the key determinant of the case burden. However, the role of immunity will become increasingly important. ‘
“Ultimately, we don’t know what the strength or duration of natural immunity to SARS-CoV-2, or a possible vaccine, will be like,” explained researcher Caroline Wagner of the McGill University. For example, “If reinfection is possible, what does a person’s immune response do to their previous infection?” Wagner asks. “Is this immune response capable of preventing you from transmitting the infection to others?” All this will affect the dynamics of future outbreaks.
The current study builds on previous research that showed that local variations in climate are unlikely to dominate the first wave of the covid-19 pandemic and included many of the same authors.
The researchers used a simple model to project the future incidence of COVID-19 cases, and the degree of immunity in the human population, under a variety of assumptions related to the likelihood that individuals will transmit the virus in different contexts. For example, the model allows for different durations of immunity after infection, as well as different degrees of protection against reinfection.
Unsurprisingly, the model found that the initial pandemic spike is largely independent of immunity because most people are susceptible. However, a wide range of epidemic patterns may exist as SARS-CoV-2 infection and thus immunity in the population increases.
“If immune responses are weak or transient, for example, larger and more frequent outbreaks can be expected in the medium term,” notes co-author Andrea Graham.
Additionally, the nature of immune responses can also affect clinical outcomes and hospital burden of severe cases. But the key question is the severity of later infections compared to the first.
Importantly, the study found that, in all scenarios, a vaccine capable of eliciting a strong immune response could substantially reduce the number of cases. Even a vaccine that offers only partial protection against secondary transmission could yield great benefits if widely implemented.
The study authors also explored the effect of ‘anti-vaccines’ and ‘deniers’ on the dynamics of future infection. Their model found that people who refuse to participate in pharmaceutical and non-pharmaceutical measures to contain the coronavirus could nonetheless delay containment of the virus even if a vaccine is available.
‘Our model indicates that if vaccine rejection is high and correlated with increased transmission and riskier behavior, such as refusing to wear a mask, then the vaccination rate required to achieve herd immunity could be much higher “Says researcher Simon Levin. “In this case, the nature of the immune response after infection or vaccination would be a very important factor in determining how effective a vaccine would be.”
One of the main conclusions of the study is that monitoring the population’s immunity to SARS-CoV-2, in addition to active infections, will be essential to accurately predict future incidence.
“This is not an easy thing to do with precision, especially when the nature of this immune response is not well understood,” says Michael Mina of the Harvard School of Public Health and the Harvard Medical School. “Even if we can measure a clinical amount as an antibody titer against this virus, we don’t necessarily know what that means in terms of protection.”