RECENT conversations on COVID-19 vaccines have flirted with the idea of mixing and matching vaccines. The argument was also touted in local circles when Jamaicans who received the Pfizer vaccine were unable to get their second doses. That problem is now resolved but the questions around antibodies and what will happen if the second dose is missed or late still remain. Below I address common questions surrounding the COVID-19 vaccine.
What if you missed your second COVID vaccine dose?
Tens of thousands of Jamaicans who had received the Pfizer COVID vaccine were unable to get their second dose when they became due 21 days later. About two months or so later, Pfizer vaccines are here again. I have often been asked whether these individuals would need to start over their COVID vaccination series. Reassuringly, there has been clear and consistent scientific evidence that spacing the COVID vaccine doses in general is associated with better vaccine efficacy. In the United Kingdom, people over age 80 years had four times the amount of antibodies against the virus when they received the Pfizer COVID vaccines 12 weeks apart compared to the originally approved 21 days apart. However, more data is needed on the effect on hospitalisations and deaths from COVID-19. It is indeed my preference that as best as possible vaccines are given based on the original approval by regulatory agencies using the data gathered in clinical trials during their development. But in practice, that doesn't always happen, and we usually try to provide vaccines at the soonest opportunity should someone miss a dose. Of course, the main concern is that during that extended period in between the doses, people would not have benefited from the full protection of the vaccines. Interestingly, though, most studies have shown that by about a couple of weeks after the first Pfizer dose, protection is about 50 per cent against infection, hospitalisation and deaths.
Your immune system is more than just antibodies
Antibodies are like Y-shaped fighter substances that our body makes and that stick to a germ, neutralising or killing the germ. But there are other aspects of our immune system that include immune fighter cells that don't make antibodies. Antibodies normally reach a peak and then fall over time after infection or immunisation. In fact, antibodies are made by certain immune cells that will remain tucked away in the lymph nodes ('wax and cannon') all over the body. These cells will start making antibodies again should you come in contact with the virus in the future or in response to a booster vaccine dose. Importantly, I have found that it may not be useful to compare antibody levels between different persons as the amount and types of antibodies produced are affected by many factors, including the person's sex, age, genetics, general health status, stress levels, etc. As I have counselled certain patients doing antibody tests after vaccination or to check if they had had COVID before, measuring antibody levels at two different time points in the same person is probably more useful information than just one measure.
To mix or not to mix?
Another common question I get is whether it is safe and effective to get another type of COVID vaccine in place of the Pfizer second dose. Only recently has such data started to emerge. For example, just in August 2021, researchers at Oxford University reported in the esteemed scientific publication, The Lancet, on a clinical trial in which they gave the AstraZeneca/Oxford (AZ) vaccine and/or the Pfizer vaccine in different combinations four weeks apart. They did not report on how these different combinations impacted rates of hospitalisation or deaths. Further, compared to other clinical trials with 30,000 or 40,000 people or more, this study had fewer than 1,000 people all aged 50 years or older. Based on these findings, for Jamaicans who had received one dose of the Pfizer vaccine, they would have had an inferior immune response with only half the antibody levels if their second dose were the AstraZeneca compared to the higher levels they would have had with a second Pfizer dose. There were no major safety concerns seen in the mix-and-match study, but the follow-up time was very short. Interestingly, for persons who had had a first dose of AstraZeneca and had gone on to get Pfizer as their second dose, they had roughly the same antibody levels as those who had got Pfizer-Pfizer. The lowest antibody levels were seen in persons who got two doses of AZ, although it was four weeks apart in that study instead of the usual eight- to twelve-week schedule for AZ.
Not the first
Canada's National Advisory Committee on Immunisation (NACI) allows up to 16 weeks between Pfizer COVID vaccine doses. Even the World Health Organization suggested that countries could consider delaying the second doses of two-dose vaccine schedules so as to try to get as many persons as possible their first dose. Those who were waiting for their second Pfizer need not worry about whether the extended interval between doses will lower the efficacy of the vaccine; if anything, they may have a more robust immune response than those who had had a shorter interval between doses.
Dr Yohann White is medical director at Para Caribe Consulting. Visit their website at ParaCaribe.com or on social media @ParaCaribeJA. Also, e-mail Dr White at email@example.com