Achieving herd immunity against SARS-CoV-2, the virus that causes Covid-19, depends in part on how long the vaccines protect you against infection. If a vaccine continues to work well over a long period of time, it becomes easier to have a significant proportion of the population protected and to suppress or eliminate the virus1.
The presence of antibodies against SARS-CoV-2 is used as an indicator of immunity, with higher levels indicating greater protection. Once antibodies drop below a particular threshold or disappear completely, a person is at risk of reinfection. There are two ways for antibodies to be generated in the body. The first is from infection by SARS-CoV-2. Initial studies observed that survivors’ antibody levels decreased rapidly after recovery. However, longer lasting are the antibody-producing cells in the bone marrow, which have been identified seven to eight months after recovery from infection. This is supported by several studies which have showed that in the short term, reinfection after recovery from the disease is very uncommon1.
How this immunity compares to that from vaccines is unclear. There are two sorts of vaccines in use in the fight against SARS-CoV-2. The AstraZeneca and Johnson & Johnson versions are what are called viral vector types. They use a modified adenovirus (which causes the common cold) to prime the immune system to respond to SARS-CoV-2. The Pfizer and Moderna vaccines use ‘messenger Ribonucleic Acid’ (mRNA) which instructs your cells to make the Coronavirus spike protein, preparing your immune system for an encounter with the SARS-CoV-2 virus1.
There is little data available for the duration of immunity conferred by the viral vector vaccines, with initial studies showing efficacy for up to two months. A later study found that a single dose of the AstraZeneca vaccine induced the production of high levels of antibodies with minimal waning after three months2. In addition, a viral vector vaccine used against a related coronavirus (which cause the Middle East Respiratory Syndrome; MERS) gave stable antibody readings over 12 months1.
Recent studies of the mRNA vaccines have demonstrated strong antibody activity six months after the second dose2. Albert Bourla, the CEO of Pfizer (the bloke Scott Morrison couldn’t be bothered calling) has stated that a booster dose of their vaccine will likely be required within 12 months of the second of a person’s original vaccination doses. The reason for this is that older people tend to have weaker immune responses and may need boosting. Boosters may also be necessary to increase immunity against new variants of the virus2.
The Delta variant of SARS-CoV-2 is much more transmissible and potentially more dangerous, and all current vaccines show a reduced efficacy against this variant. As a consequence, reduced protection over time could be problematic. It seems that this is the case, with a recent study showing that protection against the Delta variant waned within three months with both the Pfizer and AstraZeneca vaccines1.
People started to be vaccinated in Australia two months after many other countries started their vaccination programs3, with the first dose administered to an elderly woman, Jane Malysiak on February 21 of 2021. Of course, Prime Minister Scott Morrison was there for the photo opportunity4. That was almost seven months ago. My 87-year-old mother-in-law has had two Pfizer doses, one administered on the 23rd of February and the other in early March, and my partner received her first AstraZeneca dose in late March and the second in mid-June.
Morrison has ordered 85 million Pfizer booster doses, 60 million of which are supposed to arrive in 2022, with deliveries to begin in the first three months of the year5. By the time the Pfizer booster doses begin to arrive in, say, February, it will be about 11 months since Ms Malysiak presumably had her second Pfizer dose, 11 months since my mother-in-law had hers, and 8 months since my partner had her second dose of the AstraZeneca vaccine.
Israel successfully administered two doses of the Pfizer vaccine to most eligible adults by early June and this was followed by a remarkable drop in the number of Covid-19 cases and hospitalisations6. At that time, Israel was leading the world in the proportion of its entire population (58%) which had received two doses of the Pfizer vaccine. It is now at 63%7.
Israel has a relatively young population with about 30% of the population under 16 years of age8 (in Australia it is about 20%), and it extended vaccination to all children aged 12-16 in June and, in late July it extended vaccination to those children aged 5-11 who are at risk of serious health complications9. By a week ago, 78% of ‘eligible’ Israelis had been vaccinated10. Israel’s high vaccination rate, coupled with the various restrictions on movement and gatherings had led to a drop in case numbers from a seven-day moving average (SDMA) of about 8,400 per day in January to about 17 in early June11. At that time, almost all restrictions were eased. With the arrival of the Delta variant (via schools), the infection rate took off again in the middle of June12, and has now surpassed the previous peak and reached an SDMA of 10,050 cases per day (at September 3) and 31 deaths per day11. The reason for this is believed to be a combination of the higher transmissibility of the Delta variant, its partial resistance to neutralisation by the antibodies elicited by earlier strains of the virus, and waning vaccine-elicited antibody levels over time6.
Preliminary indications from Israel suggest that the Pfizer vaccine’s efficacy against infection and symptomatic illness from the Delta variant may have dropped by as much as 40%13. Breakthrough infections in mid-July have been much more common among those receiving their second vaccine dose in January, February and March than in subsequent months14. This suggests that efficacy starts to decline after about 4-5 months. However, it may not be as simple as that. In many countries, early vaccine recipients tended to be older, less healthy, and in higher-risk professions than those who got injected later on. That alone could make the protection that they got seem less impressive by comparison13.
As a consequence of this Delta wave, Israel approved the rolling out of a Pfizer vaccine booster dose on July 12th for high-risk groups and adults over 60. Initial studies have shown that antibody levels increased tenfold after the booster dose when compared to those after the second vaccine dose6. One recent study compared those who had received their boosters with those who hadn’t. Both cohorts were of similar size (4 million person-days and 3.4 million person days respectively) and showed a dramatic decrease in the total cases of Covid-19 (3,473 compared to 300 respectively) and in the cases of severe disease (330 compared to 32 respectively)6.
So, by the time Morrison’s booster doses arrive it is likely that my mother-in-law will have been vaccinated at least 11 months previously and my partner, 8-9 months previously. Given that Israel started rolling out booster doses 7 months after the commencement of their vaccine rollout, could this be another Morrison nightmare in the making? Could we see a dramatic increase in breakthrough infections of Covid-19 among the fully vaccinated before the boosters are available for Australians? Will Morrison again be the cause of Australian deaths, as he was with his unwillingness to obtain enough vaccines early enough?15
Israel and Australia were offered the same deal from Pfizer. Israel took it and Morrison turned it down. Now Australia, like Israel, is in a race against the Delta variant and we have been hamstrung by our poor vaccine ‘strollout’. The proportion of the Australian population fully vaccinated stands at 32%; this is where Israel was in the middle of February7, 7 months ago. As if that nightmare wasn’t enough, we may now be headed for another.