The picture around the BA.4 and BA.5 subvariants of Omicron has been really confusing in that the pair is driving up cases but global COVID-19 deaths remain at their lowest level since the beginning of the pandemic. Explaining the two components of the immune response – antibodies versus cellular immune responses – can help us understand where we are in the pandemic and future booster options.
These two subvariants of Omicron, as of July 5, make up more than half of the COVID-19 strains in the United States and are expected to keep increasing. One of two reasons can lead to a variant or subvariant becoming dominant strain: increased transmissibility or evasion of antibodies.
Although BA.4 and BA.5 could be more transmissible than other subvariants of Omicron (which is already very transmissible), this has not yet been established in experiments showing increased affinity for the human receptor or in animal models. What we do know is that BA.4 and BA.5 seem to evade neutralizing antibodies conferred by the vaccines or even prior BA.1 infection (an earlier subvariant of Omicron), which could be the reason we are seeing so many reinfections now. Of note, BA.1 infection conferred antibodies that protected against subsequent BA.2 infection, so we did not see the same spike in cases in the United States with BA.2 (after a large BA.1 spike over the winter) earlier this spring.
Okay, so isn’t evasion of antibodies a bad thing? Of course it is but, luckily, our immune system is “redundant” and doesn›t just rely on antibodies to protect us from infection. In fact, antibodies (such as IgA, which is the mucosal antibody most prevalent in the nose and mouth, and IgG, which is the most prevalent antibody in the bloodstream) are our first line of COVID-19 defense in the nasal mucosa. Therefore, mild upper respiratory infections will be common as BA.4/BA.5 evade our nasal antibodies. Luckily, the rate of severe disease is remaining low throughout the world, probably because of the high amounts of cellular immunity to the virus. B and T cells are our protectors from severe disease.
For instance, two-dose vaccines are still conferring high rates of protection from severe disease with the BA.4 and BA.5 variants, with 87% protection against hospitalization per South Africa data. This is probably attributable to the fact that T-cell immunity from the vaccines remains protective across variants “from Alpha to Omicron,” as described by a recent and elegant paper.
Data from Qatar show that natural infection (even occurring up to 14 months ago) remains very protective (97.3%) against severe disease with the current circulating subvariants, including BA.4 and BA.5. Again, this is probably attributable to T cells which specifically amplify in response to a piece of the virus and help recruit cells to attack the pathogen directly.
The original BA.1 subvariant of Omicron has 26-32 mutations along its spike protein that differ from the “ancestral strain,” and BA.4 and BA.5 variants have a few more. Our T-cell response, even across a mutated spike protein, is so robust that we have not seen Omicron yet able to evade the many T cells (which we produce from the vaccines or infection) that descend upon the mutated virus to fight severe disease. Antibody-producing memory B cells, generated by the vaccines (or prior infection), have been shown to actually adapt their immune response to the variant to which they are exposed.
Therefore, the story of the BA.4 and BA.5 subvariants seems to remain about antibodies vs. cellular immunity. Our immunity in the United States is growing and is from both vaccination and natural infection, with 78.3% of the population having had at least one dose of the vaccine and at least 60% of adults (and 75% of children 0-18) having been exposed to the virus by February 2022, per the Centers for Disease Control and Prevention (with exposure probably much higher now in July 2022 after subsequent Omicron subvariants waves).
So, what about Omicron-specific boosters? A booster shot will just raise antibodies temporarily, but their effectiveness wanes several months later. Moreover, a booster shot against the ancestral strain is not very effective in neutralizing BA.4 and BA.5 (with a prior BA.1 Omicron infection being more effective than a booster). Luckily, Pfizer has promised a BA.4/BA.5-specific mRNA vaccine by October, and Moderna has promised a bivalent vaccine containing BA.4/BA.5 mRNA sequences around the same time. A vaccine that specifically increases antibodies against the most prevalent circulating strain should be important as a booster for those who are predisposed to severe breakthrough infections (for example, those with immunocompromise or older individuals with multiple comorbidities). Moreover, BA.4/BA.5–specific booster vaccines may help prevent mild infections for many individuals. Finally, any booster (or exposure) should diversify and broaden T-cell responses to the virus, and a booster shot will also expand the potency of B cells, making them better able to respond to the newest subvariants as we continue to live with COVID-19.
Monica Gandhi, MD, MPH, is an infectious diseases doctor, professor of medicine, and associate chief in the division of HIV, infectious diseases, and global medicine at the University of California, San Francisco.
A version of this article first appeared on Medscape.com.